To clarify... there is no output from the video card? Also... did you mean PSU? was the one you fitted more or less watts
your question is too vague. can you please tell us more about the issue?
1. Do you hear a short beep when you press power which will indicate that all your devices are okay.
2. Did you check if the hardisk cables are properly installed. or did you check if the selector for Master/slave/Cable select is properly set?
etc. there's more things to check.
I hope you find solution for you problems.
Issues like this can usually be caused by the Processor overheating or the GPU overheating.
Incorrect drivers for your CPU will not help either.
Also, When you said changed the PSU did you change it to a higher Wattage or lower Wattage?
If so, What is the difference in Wattage now?
does it actualy reboot? or does your screen just go black?
check if all the cables are still secured.
you may have unplugged one or made one lose contact when changing the power supply.
also check if your video cable is secured properly.
I think you screen is just not getting power or signal.
need more info:
1. model of the motherboard
3. watts of the CPU
4. beep sound
remember some motherboard requires the extra 12v aside from the 20 or 24pins that may have been overlook