Loading a session file is a complex operation that can sometimes go wrong.

You have an alternative when you're stuck with a session file that no longer opens: you can usually load all the model objects from the session file.

Loading only the model objects is less error prone, and as such usually works when the session cannot be restored.

This is how to go about it:

Click the Import button underneath the top-right object list:

Change file type to ORSSession:

All the model objects found in the session file will then be loaded and visible in the objects list on the right side panel.