My computer was taking too long to start up, which I interpreted as failing to boot, but in hindsight was probably just my hard drive being slow. So, I booted into recovery mode, and ran an update. At one point, apt said “there are unnecessary packages” and would I like to remove them? I figured that apt knew better than I did (after all, maybe a package dropped a dependency), so I said yes.
It was after I noticed the very large number of packages that I suspected I messed up. Turns out, apt uninstalled the entire desktop environment, and network manager, so I had to boot into a USB drive with Network Manager installed, chroot into my main drive, and reinstall plasma. As a bonus, I think I missed the main group for the plasma desktop and only installed only most of it, so some of my extensions just didn’t work anymore.