I was playing Guild Wars 2 and happened to switch via the Windows Key to my desktop so I could access email, when the monitor began flashing rapidly between the GW2 environment and the Windows 7 environment and despite my attempting to use the mouse to close the game, I found myself locked out. After twenty seconds of frantic attempts, including ALT+CTRL+DEL to regain control over my computers environment I was no closer to solving this issue, so was forced ultimately to do a hard reset of the machine, wherein it re-started and I had a normal Windows 7 (64-bit all patches and updates in place) environment once more.
This is not the first time day that I have been forced to re-start the machine while playing GW2, and the last instance also caused the machine to become unresponsive but in that instance, was able to get ALT=CTRL+DEL to work, yet it shows GW2 was off, yet the game was clearly still running in the background.