|i'm assuming you verified that the CPU sockets are the same.|
it's possible that you didn't properly apply thermal grease and the Heat Sink/Fan (HSF) when you installed the new CPU. If this was the case, though, i would **think** that you'd have had trouble putting the old CPU back as well. Did you use thermal grease? did you clean off the old residue first? did you apply too much?
The only other way to test without spending more money is to unplug all the unnecessary peripherals from the Power Supply Unit (PSU). Disconnect all Hard Disk Drives (HDDs), use onboard video if supported and take out the video card, remove all PCI cards (sound, network, etc), and disconnect your optical drives (CD/DVD). You should only have a PSU, Motherboard (mobo), CPU, and RAM. With this config, you should be able to get into the BIOS.