Every now & then when using Vim, you'll accidentally stumble into Ex mode:
Entering Ex mode. Type "visual" to go to Normal mode. :
Nothing works the way you'd expect, many commands stop working, and the screen doesn't redraw. You will quickly go mad. Your only escape is typing 'visual' to get the hell out of there.
Chances are, you've ended up in the pit of Ex because you accidentally typed
Q. Probably because you had the caps-lock key on. If you want to free yourself from Ex forever, add this to your
nnoremap Q <nop>
That will make Q harmless, for those times when you accidentally press it.
Ex is a bit like vi-compatible mode - we struggle to think of a reason why it still exists, except for backwards-compatibility. But if you've ever deliberately entered Ex mode - do let us know...
:help Q and
:help <nop> for more.