This happened to me recently. The problem seemed to be that I got an email that my droid didn't like. I'm not sure what was wrong with it, Outlook liked it just fine, but it messed my Droid up enough that I couldn't open ANY emails from ANY account.
Delete your inbox messages or move them out of your inbox to a different folder. Refresh your droid and see if it works. If it does, then you can move the messages back (or undelete them) a few at a time until you figure out which message is the bad one.
The bad one for me had attachments ... and again, seemed to be just fine, but there was SOMETHING that the Droid didn't like. I'll be curious to see if this is the problem for you as well.