If the fluid's clean, it's not a friction element(s) welded itself together. Parking pawl is in the tail housing but never heard of one of them doing this. Anything failed to cause that will be in the back half/geartrain I would guess. If you start the engine and drop it in gear, does it act like it's in gear or does it free-rev?
If the results don't match the theory, change the theory.
