I'm sorry but that is programming speak for "dead". It basically means that Revit can't read the data because something was corrupted.
You can:
Submit a request for support to Autodesk, you are entitled as a subscription holder for Revit. You'll send the file to them and they will attempt to recover it.
Restore your project from an earlier backup file and continue. If you saved successfully within the last hour or so, just find the file that has the four digits added to it like this:
projectname.0004.rvt
These extra digits indicate the order of the backup files. Revit default to 3 backup files and will cycle through these three as you save during the day. If you start a project from scratch today and save 10 times you'll end up with the following in your project directory:
projectname.0010.rvt (most recent)
projectname.0009.rvt
projectname.0008.rvt (oldest backup)
projectname.rvt (THE project)
Good luck!