Pc get blank automatically

Friends I have 2 problem please HELP me
1.
I have Acer PC. After few minutes of working it get blanks automatically. When I start again it ((shows no signal with. all fans running inside as well as mouse and keyboard. Disc drive light blinks, one more light blinks but main power light in off. In back side there the indicator for lan connection doesn't blink although lan connected.))

2. When there is power cut suddenly in pc. Next time when i start i face same condition as above in. ( ...)

Blank refers to no power in cpu

After 2-3 hour when I start it get started in both cases.
Plz help