[Raistlin]

The mirror on that model is apparently designed to lock if the shutter is activated with flat batteries.

You might be able to free it/reset the mechanism by installing new batteries and then cycling the on/off and check/timer mechanisms.

Failing that I've found a poster on the Internet who (without sufficient detail to bother copying the post here or linking to it) describes removing the base plate of the camera and manually manipulating the shutter release slightly (which is on the right at the base of the camera) which caused the mirror/shutter mechanisms to free and complete their cycle.

If that succeeds you may need to have the camera serviced to prevent it re-sticking, if it doesn't then you might need to get it professionally fixed anyway - normally this will require a small current to be run across the terminals on the inside to stimulate the shutter/mirror mechanism to reset.