I once encountered a D50 that had managed to get the mirror stuck in the up position. The mirror had gotten just skewed enough to hang on the side of the light box. It was a very delicate task to get it unstuck without damaging the mechanism that retains the mirror when locked up or for longer exposures. Damage that latch and the mirror will droop into the light path for longer shutter speeds and bulb exposures. In that case, though, the camera still turned on and off while reporting an error.

The fact that you're asking the question here likely means that you need to send the camera out for service. It is way too easy to do more harm than good messing around inside the light box of a DSLR if you don't know what you are doing. In your particular case it sounds like the root problem may not even be in the light box. The mirror being stuck in the up position may be a result of a control issue buried in the electronics of the camera. You may even have a main power board failure. They're not that rare in DSLRs, particularly in mid-level Nikons at one time. The D40, D60, and D70 all seemed to have more than their share of power board failures. I'm nowhere as near familiar with the current Nikon lineup, though.