The company where i work have a dell desktop 780 optiplex on xp recently the computer keeps rebooting on the windows xp loading screen then i formatted to windows 7 now it randomly shuts down after some hours of usage. But the funny thing is that it doesn't really shut down it acts like its on sleep mode i.e the power button light still stays on, dvd drive still operates, fan still rolls every thing is like its working but it never comes back on.
It is not the monitor cause at that point i tested it with a second monitor and nothing comes up until i do do a hard restart that is i unplug the power cord.
I have also formatted the PC again and is still the same.
Please help i need to get this PC working

First of all get a good stabilizer to boost the voltage of the power supply as Dell desktop required constant 220VAC with no fluctuation.

If the problem persist after you are sure that you don't have low voltage in your place, follow the step below

Disasseble your PC and check the wire that go from mother board to Power button. There must be a short circuit on one of the front panel button I.e, check all the from panel pressing button you will see that some are constantly or parmernently touch/pressed.

Use screw driver to widen the space between the front case button & pannel button