I have two suggestions:
There's been some buzz on forums about a mal-formed spam message that apparently crashes Entourage's junk mail filter. You might have that pending on your mail server. So, you might try turning off the junk mail filter. If you can access your mail from a web-Mail interface or another program and remove that particular problem eMail, things might work.
Also, a generic cure for many of Entourage's ills is to rebuild its database. You can do this by a launching Entourage and immediately pressing and holding down the option key until a dialog appears asking what kind of rebuild you want (pick Advanced/complex).