I have the 645 Pro (non-TL) which is almost the same camera. At least the mirror lock-up lever is the same. On my camera I need to consciously turn the lever as the mirror lock-up seems a mechanical mechanism. I don't think that I could not accidentally lock-up the mirror so I think that there's something else going on. Could it be that the self-timer that was accidentally switched on? I've never had any trouble with the mirror on my camera. Just a wild guess; try cleaning all the electrical contacts on the camera and the back. It is an electronic camera after all and miscommunication between the various parts might give strange behavior.

In order not to waste a frame, you can put the dark slide in the back and take the back off the camera, then do what you do now to get the mirror back.

Menno