Steven,
It is recommended to save files on your own harddisk, then copy elsewhere if needed.
Concerning the recovery itself, if we are talking file corruption:
One thing often tried first is to create a new document and File>Place the (PDF contents, if any, of the) corrupted one to see how much may be rescued that way.
Here are some websites where you can see whether it can rescue the actual file, and if it can, you may pay for a subscription to have it done,
http://www.recoverytoolbox.com/buy_illustrator.html
http://markzware.com/adobe-software/fix-illustrator-file-unknown-error-occurred-pdf2dtp-file-recovery/
http://www.illustrator.fixtoolbox.com/
http://www.erepairillustrator.com/
As far as I remember, the first one is for Win and the second one is for Mac, while the third and fourth one should be for both.
Here are a few pages about struggling with it yourself:
http://daxxter.wordpress.com/2009/04/16/how-to-recover-a-corrupted-illustrator-ai-file/
http://helpx.adobe.com/illustrator/kb/troubleshoot-damaged-illustrator-files.html
http://kb2.adobe.com/cps/500/cpsid_50032.html
http://kb2.adobe.com/cps/500/cpsid_50031.html
http://helpx.adobe.com/illustrator/kb/enable-content-recovery-mode-illustrator.html