Recordable CD-Rs have a life span. Depending on the CD-Rs and who has made them, the life span can be unexpectedly short. CD-R storage is also of paramount importance. The place where you keep your CD-Rs should not be exposed to direct sunlight, CD-R should be stored vertically, etc..
After the life span, you cannot do anything to recover the files on those CD-Rs. To my knowledge it is not the files but the material from which the recordable CD is made that corrupts.
Next time, copy from time to time all information on old CD-Rs to new CD-Rs before CD-Rs become corrupt.
For your information, gold recordable CD-Rs have a very long life time.
Same is true for DVD-Rs, DVD-RW, etc..
Regards
SuatCINI