Make a complete backup , then delete the Library (and possibly Temp) folder inside the project (as peer folders to Assets and ProjectSettings), then try and open it. Unity will reimport everything.
If it’s actually a collab issue, I have no idea.
You can also try and resurrect a fresh instance of the project from collab, if it can do that.
I highly recommend you consider using an enterprise grade source control instead of collab.
Personally I use git because it is free and there are tons of tutorials out there to help you set it up.
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 the right .gitignore file:
Generally setting Unity up (includes above .gitignore concepts):