I just wanted to quickly reply as this happens to me all the time. I’m sure many of the posted “fixes” work but for me I simply command tab to desktop, command back to GW 2 and its all fine (in fullscreen mode, no need to go to windowed mode)
I’ve only run the client on Mavericks but I’d be surprised if the command tab dance didn’t work on other versions as well.
I’m in the habit of checking by rotating the toon in character selection these days to take care of the problem before I log a character in.