CD-ROMs maybe the most popular choice for storing data especially among the home consumers but they are certainly not the most reliable backup medium.
Some experts believe that the average life of a CD written with the standard CD burner is anywhere between two and five years while the factory burnt CDs have higher life. The life of CD's may also depend on other factors like the storage environment, surrounding temperature, humidity, multiple rewrites and how often the CD is accessed by you.
CD disc can easily get damaged by finger scratches, dust, incorrect handling or if someone tries to label the CD with a pen on the shiny side. (Learn how to Clean a CD)
Whatever be the reasons for disc generation, it should be kept in mind that your CDs won't last forever. To see this for yourself, find an old CD that was bundled with the first Pentinum-I computer you bought in 1995. It might throw some unpleasant suprises.
While the best [though impractical] way to remain safe is to keep testing your collection of CDs at frequent intervals or taking backup of CD's itself on other CDs or external hard-drives, the big question is what can you do if the data contained in an old CD suddenly becomes inaccessible ? You ejected the CD to clean it with a cotton cloth but unfortunately, those precious family photographs on the CD are still unreadable.
Don't lose hope yet. If you are lucky, those important files in the corrupted CD can still be retrieved with some of these free data recovery software:
CD Check - detects and recovers damaged files on CD-ROMs. CDCheck can also be used to check the health of your existing working CDs to ensure that the data residing inside is safe.
Unstoppable Copier - Available for Windows and Linux, this free utility can copy files from disks with problems such as bad sectors, scratches or that just give errors when reading data.
Normally when your computer is unable to copy a file from a damaged disk it will abort and delete the portition of the file it has copied. This program however will continue to copy the file right to the end. Any data that is not recoverable after many retries is replaced with blanks.
IsoBuster - It supports all CD and DVD formats. Isobuster has tons of data recovery features and the basic version is free of cost.
Final Advice: If you notice a scratch on the surface of a CD or DVD, immediately duplicate that disc before inviting problems in future.
Related: How to Clone Your Hard Disk