I've had a number of reports of this, all involving moving older (9.0.3 or before) databases to the current version. In all cases, this has solved the problem:
1. Quit and relaunch Bookends.
2. Repair the database -> it should now open.
3. Rebuild the database.
It doesn't matter -- if you have groups, you'll probably want to keep user settings. As noted in another thread, you won't see the groups appear immediately after the rebuild; you'll have to close and reopen the database for that. This cosmetic bug will be fixed.