Corrupted drawings unfortunately occur for a wide variety of reasons such as a bad network or hardware, operating system issues, media degradation like files on old disks, unexpected data or defect, a 3rd party application, or a non Autodesk product saving a DWG file. Here are a couple solutions to assist in recovering the files.

Recover damaged drawing files
http://support.autodesk.com/getDoc.asp?id=TS67106

Difference between AUDIT and RECOVER commands
http://support.autodesk.com/getDoc.asp?id=TS54343

Recovering a damaged drawing if the RECOVER command is not successful
http://support.autodesk.com/getDoc.asp?id=TS19369

Using DesignCenter to recover a drawing and retain layouts
http://support.autodesk.com/getDoc.asp?id=TS43354

Cheers,
-Shaan