deykuzorAre all the power connections to the motherboard put in correctly? Also are the fan headers correctly seated? The thing about a CPU installation is you're right, that shouldn't prevent a boot from occurring, but it's possible something was jostled out of place during installation.
Not entirely right. BIOS updates are required when installing a newer generation processor in an older generation motherboard even when sockets line up. However he's actually using an Ivy Bridge chipset and not a Sandy Bridge one, despite how he used to use a Sandy Bridge CPU, so that shouldn't be the issue. Plus it wouldn't explain how it won't boot with his Sandy Bridge CPU anymore either.
Bent pin maybe? And quintuple check all of your power leads. If nothing works try a different PSU.