emachine problem

i have a w2888 emachine.it worked fine all night and i shut it down properly before i went to bed. the next day when i tryed to push the start putton it wouldnt do anything. no booting up, no sounds, no lights,nothing.
i took it too a friend who is good with computers.he said it was the mother board and replaced it. after he did that the computer boots up fine, but only works for about an hour then just shuts off.
so now after he has been trouble shooting that problem for 6 weeks, he finally came it back to me and said the processer is bad. so know i have a computer from this guy that only works for an hour then turns off….i nee help please what do i do