Older ATI video chipsets seem have this problem from time to time, especially the on-board graphics ones. On one machine I have, it seems to be just the HDMI port that goes out and doesn't come back until hard reboot, after a sleep/wake cycle. Can't use the HDMI port because of it, have to use the DVI-D or VGA instead.
Then I have this monitor at work that won't work with a particular machine very well. In order to get the display working again, we have to plug the monitor into a different PC, let it display, disconnect it, THEN plug it into the machine it actually belongs to... only then will the display come back. Explain that one! I think that's a problem with the monitor, not the PC.
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