Banks carry insurance for this sort of thing. A friend of mine had over $20,000 stolen from her homeowner's loan account when a cleaning lady took two checks from the back of her checkbook drug dealer cash them.
File a report with the police. The bank will then have to sort it out since since it is not your signature on the checks. They do not check signatures on any checks anymore - go figure.

Of course, now you have bounced checks and check fees and a damaged credit rating.

Sorry.
M.