Check to see if the light baffle at the back of the mirror (near where it would hinge) has broken off, and is obstructing the mirror, preventing it from rising completely. It will look like a narrow strip of plastic stuck in the mechanism. You can also take out the focus screen to get a better view from that side.
The lens key completes it's rotation, so the body is mechanically completing it's movement. That means the spring that pushes up the mirror is probably working, and something (either in the mirror box or in the mechanism) is preventing the mirror from completing the swing.
For reference, this thread has a picture of the baffle (second page for close-ups) :
(there was a url link here which no longer exists)
If not this, then something else...