When you double-click (open) a contact in Address Book, is the email address in the Email box? If it's in another box, it will show up in autocomplete, but not in the Address Book window. This might have happened when the addresses were improperly imported from another program.

In the addressbook window I don't see anything. It is completely empty. But when I start typing an emailaddress in the To: field I can select an address as useual. Thunderbird also knows the names of my different addressbooks. I don't know what happened because this happened overnight and I did not change anything in my addressbooks for a long time.

When you say "completely empty", can you see the icons for Personal Address Book, Collected Addresses? If not, make sure View/Layout/Directory Pane is checked (press Alt to show the Menu Bar).

It is a littler difficult for me to describe this problem because English is not my native language. Therefor I added a screen-dump to make clear what I mean with completely empty. As you can see the window exists with its menubar, but contains no data about the addressbooks.

OK, the directory pane is empty. First, start in safe mode (hold Shift when you launch TB) and see if the pane looks correct. If it does, delete localstore.rdf (see below).

If it's still empty, open a Write window (new message), press F9 and see if the Contacts sidebar displays your contacts. If it does, delete these files from the profile folder; they will be automatically recreated when you restart TB: directoryTree.json , foldertree.json and session.json. See the paragraph near the bottom of this article, beginning with "If deleting localstore.rdf doesn't help...".

http://kb.mozillazine.org/Toolbar_or_windows_layout_not_working

To delete profile files, select Help/Troubleshooting Information, Show Folder, exit TB, then delete the files in Windows Explorer.