monitor on pc not staying on.

hp p2-1317cb I turn the computer on and the screen says scanning and then says no signal. I hooked the monitor up to a different desktop and it worked all right. I took the one from the old desktop and hooked it up to the one with the problem same result. I tried different cables, exchanging monitors, nothing seems to change. it is windows 8.1 and for a while it has not been coming back from sleep mode. you would have to shut down desktop and restart. I think it is a setting but I could be wrong. any ideas? don't know what to try next. thanks in advance.

 

I unplugged the main power supply and powered down and plugged back in and it came on saying doing updates, do not shut off computer. so I don't know if the updates were keeping it from coming up or not. will check it out and come back for further help if necessary. thanks

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