TLDR: After powering on and choosing my OS (I have Win10, Ubuntu, and KDE Neon all on the same disc and every time my computer starts grub(?) gives me the option between the three. Black startup screen where I can see and move my cursor. Everything else is black. I can get into tty but it just has "Login:" and I don't remember my username
This may have been caused by two things:
For a while now the KDE login screen wouldn't appear, it was some factory default login screen that I could still log in with but had some problems. After I logging in I'd be taken to my desktop and everything would be fine from there.
In the bottom left of default screen it said had in red: "The current theme cannot be loaded due to the errors below, please select another theme" and below that, a couple lines, the last of which was: "file:///usr/shar/sddm/theme/breeze/componenets/Clock.qml:22.1: module "QtQuick.Controls" version 2.5 is not installed"
So I did something along the lines of
sudo apt install qml-module-qtquick-controls (it might've been a slightly different package name, don't exactly remember). That didn't cause any immediate problems but that was the last time I'd been able to use KDE Neon: I shut down and ever since have been getting the black screen upon startup.
Another less likely thing is that I have 3 partitions for Windows 10, Ubuntu, and KDE Neon on one disk. I filled the Windows 10 partition up until there was only 3GB left. After seeing that I couldn't get into Neon I deleted ~10GB off the Windows, didn't do anything.
The main solution I've seen for this is pressing Alt+Ctrl+F*, where * is a number between 2 and 6, but that immediately asks you to login and I don't remember my username apparently.
Any help is appreciated!