Attention, No Video Input

Hey guys, I have two pcs and the one I use daily was working perfectly and all but then I tried putting the cables of the working pc to another pc which was staying now for quiet some time unused just wanted to see if it still works and if it needs some clean up and stuff, but soon as I put on the power cable and the video cable on the spare pc the monitor wouldn't work anymore it just would show Attention No Video Input I tried another spare video cable that I have but it still shows the same message on the screen and not only while using that spare pc but now it shows the same message even when I put the cables on the first pc which was working perfectly earlier.
I tried the push the power button for a minute or pushing F8 but the monitor still won't work it just shows Attention no Video Input and goes to sleep, please help!
 
Need both of your PC system specs to help you out properly, are you using onboard or dedicated vga cards in either system ?
 
Maybe you havent connected all the psu cables into your system like the atx cable that goes somewhere near the cpu fan connector. It has between 4 and 8 pins
 
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