Every user has had trouble with a scratched CD. In the case of data CDs (CD-ROMs), the drive cannot properly read the CD, thus giving rise to reading errors. In the case of audio CDs, the CD skips when we play it.
The first thing to do when coming across a CD with a read error is to clean it in order to check if the error is being caused by a dirty surface. You can even wash the CD gently with a little detergent, using your fingers to clean it. (Avoid sponges, since they can scratch the CD.) If the error persists, try reading or playing the CD on another drive. If another drive (or CD player, in the case of audio CDs) gives the same result (read error) (or skipping, in the case of audio CDs), it will mean that the CD is scratched.
Looking at it against the light, the recording surface (the flip side of the label) of a CD with this kind of trouble will allow you to easily see one or more existing scratches. A CD's data is recorded on a metal layer inside of it. This is a silvered layer on commercial CDs and is oftenly golden on CD-Rs. The metal layer is inset in a transparent plastic covering (polycarbonate) used to protect the CD's metal layer and to allow printing a label on the side not being used for reading.
A CD-ROM drive or CD player utilizes a laser beam, which crosses the plastic layer and reads the metal layer. If the plastic layer is scratched, the beam will be unable to pass through it, resulting in a read error or skipping the music. In other words, if the data to be read are still in the CD, the trouble lies in the layer of plastic.
As the CD's contents are preserved, a scratched CD can be recovered by polishing its plastic surface. After carrying out the above cleansing, if the CD persists in giving reading errors, just polish the CD with toothpaste. That's right, toothpaste. It works wonders, and you won't spend a fortune buying professional cleaning kits. Polish the scratches with a cotton swab, gently rubbing the paste-imbued swab over the scratches until they disappear or until you notice that you have removed them as much as possible. Sometimes the paste may cause new scratching, but it will be merely superficial and easily removed. After clearing the scratches, wash the CD in water.
If there are still scratches that the toothpaste has not managed to remove, use a metal polish (Brasso) in the same way as described above. Finally, rub Vaseline on the CD, very gently (do not press hard), moving outwards from the center to the rim.
The last step will be testing the CD. If it starts working properly, great. If not, repeat the above procedure, looking for the scratch that is causing the error and concentrating your polishing efforts on it.