This is driving me crazy. Ever since I installed ReSharper 4, F12 no longer seems to work. If you look at the all the ReSharper short cuts in the Goto sub menu Declar
My problem was that I couldn't use the command "Alt + F12" when trying to "peek definition" in Visual Studio. I found out through another forum-thread that it was Nvidia's GeForce Experience that overrides some of the "Alt" shortcuts and uses them for their functionalities with screen recordings etc.
What solved it for me, was going into the GeForce Experience settings and switch off the use of in-game overlay:
Alternatively, if you want to use the overlay, you can just change the short cut commands.