Can't reopen OUtlook

[coopfab]

We're running Win server 2003 sbs with xp prof. workstations. I just
migrated to Outlook from Outlook Express and the transition went fine. At
some point Outlook closed down and now can't be reopened. The error message
states, " Cannot start Microsoft Office Outlook. Unable to open the Outlook
window. The set of folders could not be opened."

What do I do?

Thanks for your interest and help.

M. Cooper