Did you properly source control 100% of your metafiles along with the project? That’s not optional, and failing to do so will result in precisely the behaviour above.
Running out of hard drive space is another “all warranties are void” situation.
Some info about Missing script warnings, GUIDs, renaming GUIDs, etc:
EVERYTHING in Unity is connected to the above GUID. It is super-easy to inadvertently change it by renaming outside of Unity. Don’t do that. Instead:
close Visual Studio (important!)
rename the file(s) in Unity
in Unity do Assets → Open C# Project to reopen Visual Studio
now rename the actual classes, and MAKE SURE THE FILE NAMES DO NOT CHANGE!
If you are NOT using source control while you do this, renaming files is an EXTREMELY dangerous process. Use source control at all times so that you can trivially revert if you miss a critical step and damage your project.