I think you are out of luck in terms of getting the messages back on your bb but try this:
-move the email to a different folder within your email account.
-wait for a few mins.
-move it back to the inbox.
This trick works on Lotus Notes accounts but I'm not certain if it will work on a BIS setup but its worth a try. The key is to trick the server into thinking that its a new email. If that fails, worse case is to forward the email back to yourself in your email account. You'll still have the content of the email but the reply-to would point back to your email account.
Also, is your bb running low on memory? If your call logs or SMS were also deleted, this is likely the case.