It's fixed, but now I'm just as confused as when I started. I did what you all suggested and unplugged everything and booted, and it worked fine. Then I plugged in everything one by one and booted fine each time. The strange thing is, I had already tried rebooting after completely leaving it and turning off the power and it didn't work then.
Maybe Levent was right and it's an overclock or power delivery problem? Either way, I'm glad it's sorted out.
Thank you all for the helpful comments, it made sorting through this mess a lot easier