I've had this same problem, but only when I use ALT+Enter to switch EQ to Windows Mode. I will switch out so I can load up my Internet Browser to look up some info on a quest/item/zone. Sometimes after I close up the browser, and switch back to EQ the screen is blank, but I can hear the sound and such (is this haapening to you too?).
Usually it just takes using the ALT+Enter switch to get the EQ screen back up properly. If it's happening while you're AFK, then I would suggest trying to switch it into Windows Mode and then back again. Also sometimes you need to ALT+Enter twice to get EQ back to fullscreen.
Also, I think the ctrl+alt+del combo only works while in Windows Mode. And beware of this, because doing this puts all programs running on your computer at the time on a sort of "pause", and can therefore crash your EQClient (won't ALWAYS happen, but CAN).