Entourage stores its database in the Microsoft User Data folder inside your
Documents folder. Inside that MUD folder, you'll see a Office 2004/v.X
Identities folder, which should contain all the relevant data. To restore
your old data, you will have to quit all currently running Office
applications and disable the Microsoft database daemon (use Activity Monitor
in Applications/Utilities). You can then replace your new data in the folder
I've just mentioned with the old one, and the next time you launch
Entourage, you should have access to your old data again. Note that this
procedure will most likely erase your new data if you have used the same
name for your identity, so if there is anything you need to save, export
your data first. Now, you didn't mention which version of Entourage you're
using, which is crucial in this case, so if you need more info, post back
and give me some more details, including the kind of data you want to backup
from your new identity (mail, calendar items, etc.).