Project spontaneously broken completely

My project was completely fine until I reopened it earlier today, after nothing having changed while it was closed. There is an error that reads "Copying assembly from 'Temp/UnityEngine.TestRunner.dll' to 'Library/ScriptAssemblies/UnityEngine.TestRunner.dll' failed".

All of my materials are now using "Hidden/InternalErrorShader" as their shader.
All of my scripts simply say "The associated script can not be loaded. Please fix any compile errors and assign a valid script."

The collab window doesn't even open.
I've tried reverting to backups, which didn't work.
I've tried doing reimport all, which didn't work, and crashed the editor.
I've tried updating to the most recent stable release, which didn't work.
I've tried deleting the temp folder, and deleting the library folder, which caused it to crash on the packagemanager reaching the VR module.

How do I fix my project?