Seems to be related to this question What is the '[Command Line]' that sometimes comes up when I try to quit, and how can I quickly exit it?, but the solution doesn't work for me.
I pressed esc, typed :q, <C-w> q but nothing seems to work. The window seems to capture every input and simply displays it, even mouse input!
This has happened to me a few times and everytime I forced to close the terminal. How do I go back to my work without closing vim and without closing the terminal.

:iinsert mode by pressing escape. Next time it happens, I'll try the exit keys you showed and see if it helps. – Samaursa Mar 14 '18 at 15:16