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Anticlimactic and somewhat embarrassing update: as some people suggested, I left it unplugged for about half an hour last night and then tried plugging it back in... and it didn't work. So I left it unplugged for a couple of hours and then tried it again before bed... and it didn't work. Same error light despite multiple power-cycling attempts. So I left it unplugged overnight and plugged it back in today to try some of the BIOS stuff that other people suggested... and it booted up immediately without issue.

I feel stupid about even posting this now, especially since it blew up a bit, but I was tired and irritable after a long day of work, and an ominous GPU error code wasn't exactly the seamless plug-and-play experience I had hoped for. But I guess if anyone encounters the same error, don't panic like I did, just let it sit for a few hours and it will somehow sort itself out. Anyway, I'm sorry for the false alarm, thanks to everyone who suggested solutions, and now I'm going to spend this weekend playing Crusader Kings until my eyes hurt.