I had a problem with my D750. It worked fine until one day after turning it on, the mirror would flip up and stay up. I wound up having to press the shutter button a second time to get it to drop back down. Then it worked fine until the next time I turned the camera back on. And it kept happening each time the body was turned on. When searching for information, my issue has to do with the CPU. But since I'm not a fan of Nikon's repair service, I found a workaround that seems to be okay.
Before turning off the camera, I wait until the top display goes into standby. When I do that, the camera operates normally when turning it on the next time. Since then I've rarely encountered the issue.
There is a reset that Peter7100 posted in the past. No clue whether it will work for you. But when I was having trouble with my D7200's shutter button not autofocusing, the reset restored that body back to normal.
Re: Tip - Attached lens won't autofocus
Not sure if this is relevant to this make/model, but on Canon body, I was able to reset things by doing the following:
1. Switch off
2. remove battery and card
3. switch on
4. Hold down the shutter for at least 15 seconds
5. switch off
6. re-install battery and card
7. switch on and keep your fingers crossed.
It worked for me and cleared unusual issues I was having with it.
This was apparently a suggestion from a technician in one of their factories and was not widely known in the public domain. It makes you wonder if this is what they do when people send their cameras in for a repair costing a fortune
Try at your own risk!!