One screen blacks out after long idle

I have two Radeon-cards in my computer, each is connected to a display (so I have two displays). Everything works fine but when I leave the office, leave the computer on and return in the morning one display (always the same) is black.
I can open up arandr, deactivate the display, then reactivate it and it works again. Any ideas what could cause the problem?
I don't see any obvious problems in messages, debug.log or Xorg.0.log.
Try xset q and see if one of the displays (monitors) have DPMS activated. Although if it is DPMS the monitor should turn on as soon as you press a key on the keyboard.
Sorry for the late reply and thanks. But DPMS isn't the problem.
I noticed that the problem already occurs when I switch the display off and that it doesn't come back up.