I’m sorry you’ve had this issue. You are most likely going to need to go to a backup of that scene file and restart from there. You might be able to open the scene in a text editor and chop off or cut out blocks of YAML that look bad, but that is NOT supported, NOT recommended, and would require you to revisit the entire scene anyway to look for damage and repair it.
I suggest you immediately consider setting up and using proper industrial-grade source control in order to guard and protect your hard-earned work. This is 2021 after all, and such random loss of data is well understood and can easily be guarded against.
Personally I use git (completely outside of Unity) because it is free and there are tons of tutorials out there to help you set it up as well as free places to host your repo (BitBucket, Github, Gitlab, etc.).
As far as configuring Unity to play nice with git, keep this in mind:
Here’s how I use git in one of my games, Jetpack Kurt:
Using fine-grained source control as you work to refine your engineering:
Share/Sharing source code between projects:
Setting up an appropriate .gitignore file for Unity3D:
Generally setting Unity up (includes above .gitignore concepts):
It is only simple economics that you must expend as much effort into backing it up as you feel the work is worth in the first place.
“Use source control or you will be really sad sooner or later.” - StarManta on the Unity3D forum boards