Q: Entourage content wiped out

Was wondering whether someone could help me with this.

On logging on this morning, I discover that all my emails, have been wiped from Entourage.  Calendar, contacts, to do, are intact.

Entourage is prompting me to set up the email system, ie as a new user

I've taken a look at the "main identity" folder - the database folder is 25 MB, suggesting to me that my emails aren't sitting in there!

Any suggestions??