More often the problem is with the motherboard than the CPU. Test the board with
the minimal items connected - PSU, 1 stick of ram, CPU & Heatsink, and if you don't
have integrated graphics - add the video card.
In my case, I had two sticks of ram, and one stick was dead. And this stick of ram
was keeping the entire computer from booting. I tested with one stick at a time and
it booted. I thought it was the motherboard/CPU at first, so I was lucky, I only had
to return a stick of ram.