Cannot Click Anything from inflight dropdown menu (ATC, Checklist, Weather Settings etc)

I suddenly encountered this issue that none of the menu items from the drop-down is accessible through the mouse. I even opened the ATC window using keyboard control but it froze at one place, Cannot move or click anything in the window… Rest everything as far as aircraft controls are concerned is working fine with the mouse.

Can anyone help me solve this issue?

That sounds like a window in the background has focus.