vnqaa,
If you are saving elsewhere than your own harddisk, that may be the cause. Otherwise, something else has obviously corrupted the files, possibly related to graphs.
One thing often tried first is to create a new document and File>Place the corrupted one to see how much may be rescued that way.
Here is a website where you can see whether it can rescue the file, and if it can, you may pay for a subscription to have it done,
http://www.recoverytoolbox.com/buy_illustrator.html
and another similar website,
http://markzware.com/adobe-software/fix-illustrator-file-unknown-error -occurred-pdf2dtp-file-recovery/
As far as I remember, the former is for Win and the latter for Mac.
Here are a few pages about struggling with it yourself:
http://daxxter.wordpress.com/2009/04/16/how-to-recover-a-corrupted-ill ustrator-ai-file/
http://helpx.adobe.com/illustrator/kb/troubleshoot-damaged-illustrator -files.html