It is possible that your Office preferences became corrupted and Entourage created a new profile for you. Your data may still be there. I don't have Entourage any more but believe that you should look in your account's folder:
~/Documents/Microsoft User Data/Office 2008 Identities/
If you have several versions of "Identity" folders in there, then you need to back them up immediately. Look at the modified dates of the database folders and files in there to get an idea which one may be the identity with your Entourage identity database. If you find it there, after backing it up, you can try some of the things in this MS article to repair the Identity or to have Entourage create a new one and then import your old identity into the clean one:
http://support.microsoft.com/kb/268322
Good luck.