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Is it a laptop or desktop. Powering it off and back on does nothing but have a black screen?
If it's a laptop unplug power cord and battery for a minute, leave battery out and plug it in. Then power it on.
the power saving mode is probably indicating that the monitor/cable is not working correctly. Double check the monitor cables, plug them in/out going to from computer and make sure they are tight. Do you know if it's vga (blue end) or dvi (white end) or something else?
What color are the ends of the monitor cable?
sounds like you have an external graphics card if you have dual monitors
Are both monitors plugged into the very bottom external graphics card? You probably got a Windows update related to your internal/external graphics card. Try only plugging 1 monitor in at a time to see what happens.
blue is probably going to internal graphics card, plug that in
then unplug white cable
then try the opposite if that doesnt work
I had to leave for awhile. Can you remove the external graphics card and just use the internal one?