I had this problem a few months ago when I was putting together my current PC. First I got a loud system beep and the CMOS display would stop somewhere in the 20s. That turned out to be my video card being improperly powered. Then it would boot and crash hard before getting to the OS. Turned out to be a bad stick of RAM. I’m glad I had my laptop around to look up CMOS codes and stuff, but I wish I’d had this list to check out too.
It starts with the basics (plugged in?) but moves to more advances stuff like clearing the CMOS memory, checking your PSU, and cleaning PCI interfaces. If your computer is not booting at all, this is a great checklist to run down before freaking out.