As others have already said, MDB files were used prior to A2007 but can be opened in later versions. However there is a limit to which version can open it depending on the version in which it was made
Coming back to the possibility of corruption....
First make a couple of backup copies just in case
Compact & repair is very unlikely to fix it though is worth a try
Next try DECOMPILING to remove any compile code which may be corrupted
See
http://www.fmsinc.com/microsoftaccess/performance/decompile.asp
If it opens after that, RECOMPILE then COMPACT
If that also fails, then trying copying everything into a new database
If that also fails, repeat but copying one item at a time and testing after each item, so you can pin down which item(s) are causing the issue