I'm not sure whether this is regarding a vim command or a screen command, although I think it's the latter.
I use vim under screen all the time, and every once in a while will hit a key combo that freezes the virtual terminal I'm using.
I can see everything, and I can still issue screen commands on that terminal, but I can't perform any actions within the terminal itself. I can create/flip to other virtual terminals and they all work nicely.
I've no idea what I'm doing to lock up one particular terminal.
Also, searching for answers online, CTRL-Q and CTRL-S are mentioned everywhere. Hitting CTRL-Q does nothing, and given the ambiguity of it, I tried both CTRL-q and CTRL-Q.
–edit–
One other detail is that I'm using a Macbook, but I ~think~ I've had the same problem using KDE/Mint on a PC.
Best Answer
The accepted answer talks about flow control for the terminal you are running GNU Screen in. This would not result in the behaviour you are describing since you wouldn't be able to flip to and use your other Screen-terminals.
The behavior you are describing sounds like you turned flow control on within GNU Screen itself. If you hit Ctrl+A followed by S this will send XOFF to the current Screen-terminal, but will leave the other Screen-terminals you could switch to alone. To get things working again, simply hit Ctrl+A followed by Q in the terminal that you want to get moving again.