Assuming that you have looked at the cpu and it looks fine and was reseated properly,(as3, etc.)
I've had that happen before on my setup. It is hard lock of the cmos and you should take the cmos battery out for 2-3mins, put it back in then clear the cmos . With mine I find if that doesn't work I take the battery out overnight and it works all the time. Difference was I was ocingmy system to the max it can handle, and crashed cause of that. It is unusual that it has happpened, I would reseat your atx connector as well for peace of mind.
Hope it all goes well