Sounds like a crashed database.
If you're running MySQL as the database system, please check out this post for details on how to possibly repair your database:viewtopic.php?f=20&t=28106&p=138973#p138973
Firebird isn't really maintenance friendly as there's no way to access the database daemon in the embedded version and probably all the manual has to say about repairing a database is pointed at the standalone version. Feel free to try your luck and dive deep in the Firebird website/manual if you feel it's worth it, rather than just uninstalling and reinstalling SAM and Firebird.
If you're using PostgreSQL or MS SQL, check out the manual of your database system about repairing databases or crashed tables.
But I guess people using these already know how things work.