I have the acer aspire m1100 and due to a power cut the computer went off. When I went to turn it back on I was getting the same problem, no booting of the PC.
I have tried everything, replacing the PSU, RAM, Added a graphics card instead of the onboard one, reseting the CMOS, CPU warms up.
Did you check all the cables. Go over all the cables. Was the PSU a new one or an old one, does that work for sure? Get a friend to go over all your work - its sometimes easy to make the same mistake many times. Cheers... hope you finish this :).