Have you restarted your Mac since it happened? Did it happen again since then?
If you wait many minutes, does it ever come back from the "locked up" state or is it apparently locked up forever (until you force quit it or restart the Mac)?
Are you using the application named "Mail" that came with your Mac or some other e-mail program?
When you launch it, does it give you any chance to interact with it (such as to make changes in your preferences) before locking up?