Not very long ago, I was sent a cd by someone who claimed she could not access a file she needed on one of her personally made backup cds. No matter how many times she would try copying the file to her desktop, she would get  the same error, informing her the file could not be copied due to some type of corruption. She wanted to know if I could possibly help.  I opened the cd in my own cd-rom drive and tried copying the file she was having a problem with and I realized it couldn't be copied on my system either.  Therefore, in all likelihood, it was the cd itself that was problematic, not the system, player or other hardware.  The error received when trying to copy the file was a cyclic redundancy check (CRC) error. In fact, this is the most common error encountered with cd/dvd access or copying problems and I knew that such a problem was often caused by a dirty or scratched disc, so I examined the surface of the cd and just as I suspected, it was marked.  It had a distinct smudge print on it.  I cleaned the whole cd and after doing so, voila! , I was able to copy the needed file from the cd to my hard drive.

I informed her that the problem was fixed and when I told her it was just a matter of cleaning the disc, she was surprised that the solution was so simple and never thought such a confounding problem could be caused by a less than pristine condition cd. 
 
The fact is that cds and dvds can be very finicky. Just one speck of dirt or a hairline scratch can cause problems in accessing your data or the disc itself. These marks can make it difficult for the drive's laser beam to properly read the disc. Fortunately, dirty discs can be cleaned. Scratches can be more problematic, though, in many cases they can be repaired.

With scratches, the severity of the problem depends much on the type and location of the scratch. Scratches that run across the disc or along the track, as opposed to those that run in a straight line from rim to center, can be the most difficult to repair. 
 
These type of problems are unique to cd and dvd media because the surfaces of the discs are bare and exposed, offering no protection against dust, finger prints, smudge, dirt and scratches. An exception is most dvd-ram discs which are encased in protective cartridges. Hard drives are enclosed in a tough metal shell and even cheap floppy discs are protected with a plastic cartridge. 
 
 
It should be stated that most scratches and marks do not usually affect playing a cd or dvd but when a problem accessing a cd or file does occur, the reason is most often dirt or scratches on crucial sections of the disc. For example, if the lead in area or end points is damaged, you will most likely have problems accessing the cd or dvd and receive the same CRC error as I did. Without getting too technical, a Cyclic Redundancy Check (CRC) is a method used to detect errors during file copying or transfer.  When you copy a file from your cd to your hard drive, the operating system performs a CRC on a block of data on the cd and returns a mathematical result - a number - known as a checksum which is used to identify your file. When the checksums do not match, you get the error and as a result, your file file cannot be copied or opened.  It can be a very frustrating experience. For more detailed  information on CRC. see here 
http://www.4d.com/docs/CMU/CMU79909.HTM  
 
CRC errors can also result from using low quality media which has degraded over time. All cds and dvds degrade with time and use but not all degrade at the same rate. If the data you need to store is important, you should make a point to use quality cd and dvd media, such as that made by Taiyo Yuden. 
http://www.t-yuden.com/recordablemedia/index.cfm 

In general, the dye used on discs can affect longevity. Media  based on pthalocyanine are considered to have a greater life expectancy than those based on cyanine. Standard media, as made by well known, reputable manufacturers, i is said to have a 50 year life span. This may or may not apply to cheaper no name media.

Also, becoming more common now are anti-scratch discs. TDK, for example, now manufactures, "armor plated" dvd discs which they claim are virtually unscratchable and have a life span of 100 years. 
http://www.tdk.com.au/product_DVDArmour.asp.

Kodak makes the Infoguard CD-R which they say can last 100 years, as well. 
http://www.cd-info.com/CDIC/Technology/CD-R/Media/Kodak.html? 
 
 
Solutions 
 
Ok, so what to do if you can't play your cd or dvd or can't copy a file from it. The first step, as you should now realize would be to clean the disc. 

Commercial cleaning fluids are best but I've had good results just by running warm (not hot) tap water over the disc and gently wiping and rinsing the disc clean. Make sure to use a soft lint free cloth and never rub around the disc in circular motions. Always wipe in straight lines from inside center to outside rim. Then, wipe or pat dry. 
 
Never use alcohol or abrasive household cleaners on cds or dvds.  They can damage the disc surface and most soap products can leave a residue.  If very dirty, a drop of dishwashing liquid (which rinses clean with no residue) in a quart or so of water can be used.  
 
For minor scratches that affect the ability to open or play your cds, a dab of toothpaste can do wonders. Using a qtip or cotton swab, gently polish a touch of toothpaste into the scratch to fill and smooth it out  Again, never wipe in a circular motion. Always clean from center to rim. Then rinse under warm tap water. An alternative to toothpaste that may also help is petroleum jelly (Vaseline)  Just a tiny  touch of it to smooth out the scratches and then rinse with warm water. 
 
If home remedies don't solve the problem, you can try a commercial scratch repair kit. Some effective ones can be seen here: 
http://www.allsop.com/cd-dvd-cleaners/cd-dvd-scratch-repair/ 
 
But before rushing out to buy one of these kits without knowing if you'll be wasting your money or not, I would advise an attempt to copy the bad disc to a new disc first. . I've had success doing this by using the trial program CDClone to copy a 
badly scratched problem disc to a new one.  
http://www.slysoft.com/en/clonedvd.html 
 
I would also recommend trying ISO Buster which is highly regarded and totally free. It may allow you to extract the bad cd contents to your hard drive or mount the bad cd for copying to a new cd. 

More info here 
http://www.isobuster.com/. 
 
If copying doesn't work, there are certain programs you can try that may help you recover data right  from the  damaged disc itself. One that is very good and also free is CD Check. Even if a complete recovery isn't possible, it may be able to recover a file partially.  It works on all the major PC systems from Windows 95 and up and is especially good for CRC errors.

It can be downloaded here: 
http://www.softwarepatch.com/software/cd-recovery.html 
  
Another excellent program  for corrupt disc recovery is Bad Copy Pro which works not only on cd/dvd media, but floppies and zipdisks.  For more information and to download a free evaluation copy of the program see here: 
http://www.jufsoft.com/badcopy/  
 
In closing, it should also be noted that some cd/dvd players are better than others when it comes to their ability to handle scratched or defective discs.  If you have plenty of scratched discs and frequent problems playing them, you might want to consider a new cd/dvd-rom drive that is known to excel in reading bad media.  Check out  CDRLabs for reviews and pay particular attention to the cd/dvd error correction tests. 
http://www.cdrlabs.com/reviews/