My PC won't recognize my second monitor, but it's duplicating TO that monitor

I've tried updating the drivers, windows itself, pressing WIN+P to try and set it to Extend, tried rebooting, tried rebooting with the cable unplugged, and nothing is working. It happened immediately after I downloaded a pretty large file (an 8 hour twitch vod from the twitch website, so no viruses), and I'm at a loss.