I don't know if this'll help you, Zeeshan ...but I was having this issue as well. Then I noticed NVIDIA Overlay had ALT+R as the shortcut to "Toggle Statistics" up in the corner of my screen. This was blocking me from clearing Rotation in Blender.
I switched it to Shift+Control+Alt+R in the NVIDIA Overlay Prefs and that freed Blender right back up. I was missing it as the Statistics info was difficult to see when I wasn't in a game...barely noticed it in Blender.
Not sure if you've got an NVIDIA card, but see if that works for you!
Thanks, everyone, for the help. Yes, PJZ, you are right. That was because of NVIDIA overlay settings. Also, I figured out that we can clear rotation by Selecting an object and then clicking the object in the Header, click clear, and clear rotation.