What to do if you've lost your address book(s)
The first think to do is to look in N:\mulberry using either Windows Explorer or My Computer. Can you see the address books listed? If you can then you've not lost them. If you can't, then you need to email help asking for them to be restored from backups giving exact names of the address books and when you last had them.
Assuming you've still got the file(s) but Mulberry isn't using them, you need to look in the Address Book Manager within Mulberry. To do this, start Mulberry and open the manager from the 'Address' menu.

If you see a window like below, an address book with no ticks in the diamonds, this is why your address book isn't working, tick all three boxes.

Close the Address Book Manager, you should be ok now.