It might be dead, but....
On the other hand it might not have reset properly, particularly if the battery is old and not holding any charge.
Try turning off the mains - maybe at a fused connection unit, otherwise at the fuse/circuit breaker on the consumer unit (might be on downstairs power circuit - check LED on front of main unit casing has gone out). Wait for a minute or so, then try re-powering. It might take a couple of attempts.
Had this problem myself - was also prone to falsely triggering the alarm at power on.
Ian, December 2008