It may be simple but make sure you have the object you want to clear the rotation on.
If ALT+R doesn’t work I would agree with @spikeyxxx , when I installed my new GPU recently, I had exactly that problem. The hotkeys for the graphics card took priority over Blender, I didn’t realize until I CTRL+SHIFT+S to save and nothing happened, or so I thought, I had in fact saved an instant replay. Double check your GPU hotkeys, I turned all mine off as I don’t need them, and check any other software running in the background.
