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.
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.