If it is a CD-R, it cannot be written to again, so mechanical damage is the likely culprit. If it was a CD-RW then, in addition to damage, it is possible that it was re-formatted at some point.
If it is several years old, it is possible that the dye layer is failing - much more likely with CD-RW than CD-R - although if it has been left exposed to sunlight for any significant length of time, all bets are off.
SD