The camera is confused. The interlocks are engaged in some way that makes it think one or more of the following:
1) The dark slide is in the way;
2) The film hasn't been wound;
3) The revolving back is somewhere intermediate - neither fully horizontal nor fully vertical; or
4) The lens isn't cocked.
Any of the above circumstances will prevent you from releasing the shutter.
Double check the rotating back - even if it is just a couple of degrees off of clicking into place it will prevent shutter release.
Is the "Mirror Up" function engaged on the lens? That might confuse things.
Have you read through the "Special Pointers" on page 39 of the manual?