I used the "Lock mirror up for cleaning" feature of my Nikon D5100. It worked several times, but the last time I did it, turning off the camera didn't work: it's supposed to unlock the mirror and go back to normal, but it did not bring the mirror back. After looking closely, it seems it did close the shutter (well, it's closed now, I don't see the sensor anymore).
Now, the camera is like dead (indistinguishable from a camera without battery): turning it on or off does nothing, not even the green light which normally blinks when turning on. Removing the battery and putting it back doesn't work either. None of the button have any effect.
I tried two different batteries, at least on of them was fully loaded before.
edit: I also tried loading the battery fully, insert it in the camera, turn it on/off several times & manipulate a few buttons, and the battery is still loaded afterwards (the charger blinks for ~2 minutes and then shows the battery as fully charged, just like it does for a normal fully loaded battery). So it does not look like an abnormal battery drain. Thanks @Chris H for the idea.
edit2: if I pull the mirror gently, it does move (but returns flips up immediately when I release it), so it's not just stuck mechanically in upper position.
Any idea of something I could try to get it back to life (before I take it to a repair shop)?
edit3, conclusion: the repair shop confirms the camera has an electrical problem (essentially, all the electronics inside needs to be replaced :-( ).