Compact Discs (CDs) don't last forever. They get scratches for instance and sometimes these make the CD or some sectors become unreadable. Now, what can you do if you desperately need to access the data that is on the CD? The article Recovering Scratched CDs from Hardware Secrets tries to answer this question.
As the CD's contents are preserved, a scratched CD can be recovered by polishing its plastic surface. If, after carrying out the above cleansing, 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, rubbing gently the paste-imbued swab over the scratches until they disappear or until you notice that you have removed them as far 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.
Update: We have published two application overviews that allow you to recover files from optical discs. First the CD Data recovery overview, and a year later the CD and DVD data recovery software overview. Some even let you create error correction codes that aid you in recovering corrupt or unreadable files. While you have to create those codes in advance, they can provide you with a way to recover data from CDs or DVDs that may otherwise be not recoverable anymore.
The majority of programs however try to read data from discs in various ways and methods. Some allow you to create hashes of files stored on those discs, which you can use regularly to check whether the file contents have changed, or if the files are still fully readable.
If you store important data on optical discs, you may want to consider checking them regularly and having another backup available as a precaution.