Unable to remove lens from my D750

I had this occur with my D800 and a lens I bought used from LensRentals.com. I sent the stuck lens+body into them to figure out the problem.

I don't know if I could have resolved it myself. However, there is no "force override separate the lens no matter what" mechanism. Without special tools or knowledge about what the possible failure modes are, do not attempt to force the lens removal.

I suggest you send the camera+lens to whatever service center or warranty repair that is applicable. That is the best way to both guarantee your repair rights as much as possible, and to ensure you don't cause damage that is not covered by warranty.

One of two things has likely happened. The locking pin has bent and is not retracting fully when the button is pushed. Or one of the mounting rings has come loose and bent, causing it to bind when turned.

Neither is an easy/pretty fix, and will likely require replacement parts after they have been separated (by force or cutting).

If it is only moving very little it is probably the locking pin; and you might be able to get it to retract by turning the lens back and forth while simultaneously pressing the release button very firmly. Even if you do get the pin to retract and release the lens, it probably won't return and function normally afterwards.

I would expect a loose/bent mounting ring to be warranty repair; but it wouldn't surprise me if a bent locking pin isn't. Either way, it's almost certainly best to let a shop do the work.

It sounds like the release is not retracting properly. It needs to be lubricated (VERY CAREFULLY). I suggest sending it in for repair. It will happen again if not repaired.