Black screen on boot

Hello,
Dear sir / Madam

i have face a big problem, when i turn on my pc then my cpu is runnig and botting screen is came my monitor, the when welcome screen is came to my monitor then my monitor screen is off but cpu is running, the i turn off my pc from ups directly, then when i turn on my pc second time then ok, every thing ok,
when i need to restart to my pc the my cpu is running but my monitor screen totaly balck monitr's small green light is on but so lightly not deeply like when pc is ok.

what can i do.