Mirror stuck in the up position.

I have just got my first Hasselblad second hand. Having never use one before, I fitted the A12 film back to the camera (no film in) and removed the dark slide. When I pushed the shutter, all seemed to work fine but the mirror is now locked in the up position. The camera does not wind on and the dark slide will not go back into its slot. I am being very careful not to force anything.

Can anyone give me a hint about how to reset the camera?

I would appreciate any advice.