Just noticed if I lock the computer ([Ctrl]+[Alt]+{Del], Lock This Computer), and then return, the game screen is completely black, although the cursor does change from an arrow to a hand when passing over objects that would otherwise be there and clickable. Switching between tasks does not bring the game screen back. I can only force quit the task and then restart.
try [WIN] + [L] … it has the same result but is easier and quicker ;o)
@topic: Same issue when UAC gets active because a program needs admin rights, e.g. the task manager or fraps.