I had the same issue, PPro would not load the project because I had left the Multicam Window open. I noticed my hard drive light was still flashing, like it’s loading, but very slow. I tried defrag, CCleaner, which usually fixes many issues but nothing helped.
The only thing that worked was creating a new project, and loading the original.
I think this is definitely a bug from the Multi-Cam monitor being left open upon closing the project.