I launched the launcher hoping to play some more in my world, but alas as always when I feel motivated to do something, things go wrong. It was working the day before but just randomly happened today. Something else note-worthy; I used CurseForge to launch Vanilla when the error/bug occurred, I didn't launch modded or anything before this launch.

As soon as it loaded in it displayed the message shown it the image, and I tried re-launching but it didn't fix it, I haven't yet tried to re-install or repair, but I will try them after writing this and leave a comment on how they go