Keyboard Stops working after flashing not allowed sign

I have been using Ubuntu from last 2 years and i don’t know from where this problem suddenly arises.
My keyboard is working fine on login screen of Ubuntu and lets me in. But after entering desktop environment not allowed sign starts flashing and pointer(where i have to type) starts blinking and I’m unable to type. I used external keyboard also, the problem still persists.
On Screen keyboard is also showing same behavior. I have checked it in other Linux os also, problem is there also.
I thought there would be some hardware problem, went to service center, he said keyboard is absolutely fine.
After that I installed window and this problem never happened there. But about 4 to 5 days back I switched to Ubuntu again, the same old problem arised.

Please help me in this. This is irritating. I don’t want to switch to windows.