Larry is having an issue where the computer he built is suddenly freezing up and not letting it reboot. So he's gotten into the habit of using a bootable backup. But now he can't get the computer to boot back up. Leo says that most problems like this can be solved by just reinstalling Windows from scratch and starting over. But if it's freezing during the boot, then it could be a host of things, including cabling. One key is to wait for the "beeps," or Power On Self Test (POST). If it beeps, that indicates an issue. Also, pull out your dedicated video card and try rebooting. The video card could be bad. The power supply may have also gone bad.
The chatroom suggests going into the setup at the beginning of boot and clear the BIOS. If you can't even get to that, it's likely the motherboard.
Leo also suggests trying to boot up with Linux on a thumb drive. If you do that and it boots up, you know that software is the issue. If not, then it's almost certainly a hardware issue.