First try confirming the repository folder exists for your LAB_PROJECT (look in DefaultLab_project).
Also look at the project details on the Lab Management tab in site admin and check existence of the Project directory and DB stated there.

If you have a backup of your lab project (DB and repos), you could try restoring it from backup - similarly to what you do with QC projects -- with exception of doing remove/restore on the Lab Management tab.
NOTE: RESTORE is not available with a rt-click, BUT it is a button on the button bar - it looks like an arrow circling back on itself.

Another ''trick'' to do is if you have a nonworking
Lab_Project (messed up) but you are not using it or it
is ''blank'' or expendable and you want to start over with a NEW/blank Lab_Project, you can either
obtain a backup of a ''blank'' lab_project, or stand up a test QC server (same version as you are using)
and run/rerun
the Config Wizard and create a NEW qcsiteadmin_db and it will also create a new
Lab_Project at the same time (remove the old one first from the DB and repository, or pay attention to
the popup messages telling your he naming convention when it creates the blank one). Then you can
migrate/copy the DB and repository from the test server to the other/Prod server.

Treat this backup similarly to how you would a QC project to be restored. Then the Project restore is done on the Lab Management tab.

For good measure, then you should do a VERIFY (and possibly REPAIR) as you would with any restored project.