This happens to me from time to time. Very simple and easy to fix.
Step 1: Go into GeForce Experience, open up Shadowplay and turn it off.
Step 2: Go into your start menu and search "Run", and then open up the Run application.
Step 3: Type in "services.msc" and click Okay.
Step 4: Scroll down the menu until you see "NVIDIA Stream Service".
Step 5: Right click on it and select "Restart".
Step 6: Close the menu and turn Shadowplay on.
Hope that helped!