Computer Won't restart by itself

I have a new system which I just build with the latest is been working but a few months ago, it began to not restart. here is the problem.

When I do the reastart action it would shutdown, but that is it it won't come back on nothing on the screen. no activity on the hard drives, the system is on but nothing happening.

it will restart if I press the restart switch manually. Any one has any ideas?

I saw that might be the monitor drivers but that wasn't it, lately is been blanking out. nothing on the screen, no activity is as if it went to sleep but not commin back on line, I has to reastart it if I want to use it.

ANY FIX LET ME KNOW THANKS