Did you use rewritable discs?
Re-writeable CDs or DVDs are never the best choice for archiving data. Files written to them in batches at different intervals can often become unreadable over time when software versions get updated, or the files are unreadable on a different computer to the one on which they were written. It's always better to fill a DVD in one write session and leave it alone.
There are recovery tools, but a degree of trial and error may be required. Try this one for starters.