After recent Upgrade: Older projects => No Repository

I am currently running selfhosted Gitlab CE 13.5.4 on Debian 9.13 (Stretch)
I just discovered that about 100 older projects has turned empty and report “! No Repository”
Are there any way to try recover the projects other than series of restores and rollbacks of versions?
I am also confused where Gitlab stores git data.

When I discovered that several old project turned empty (with the error message “!No Repository”) I wanted to check if the git repos was still on the server file system.
I got confused when I looked in /var/opt/gitlab/git-data/repositories/
The groups I checked with projects that has turned empty has .wiki.git files
But some more recently accessed groups have non empty projects with files that I can clone through the web interface. But the subdirectory with the group name in /var/opt/gitlab/git-data/repositories/ is empty.
Are there any resources where I can read up on the relationship between what is shown in the web interface and how it is stored on disk?
Are there any “fsck” for GitLab data that finds and repairs db/file/git structres that got out of sync?

Solved!
The cause of the “No Repository” seems to be related to the migration to hashed storage in connection with recent gitlab upgrades. I am under the impression that the migration process might be part of the upgrade and that it failed du to memory constraints.

I managed to resolve this issue in two steps.

1. I increased the gitlab virtual machine memory to 4GB
2. I ran “sudo gitlab-rake gitlab:storage:migrate_to_hashed”
   I waited patiently for about 10 minutes to be sure that all my 180 projects had been properly migrated.