I am having the exact same problem. Monitors set to sleep after 15 minutes of inactivity. Upon touching the roller ball this morning 3 of the 4 monitors come to life as usual. The new Dell stays dark, but the power button is lit. Pressing the power button does not turn the monitor off; is as if everything is frozen. Must use the power cable to the monitor to unplug/replug then the monitor works again.
GTX 670 card. Monitor hooked up hdmi-hdmi.
Running at the 1920X1080 resolution.