1) Try a shutdown and reboot. (data files could be held open by something else).
2) Try disk test (could be disk corruption).
3) Try a drive clean (too many junk files can block Outlook from running, as can lack of disk space).
4) Check your Outlook .PST file size, for versions of Outlook up-to 2002 there is a limit of 2Gig, latter versions it's 20Gig.