So you lost a thumb drive containing a git-annex repository. Or a hard drive died or some other misfortune has befallen your data.

Unless you configured backups, git-annex can't get your data back. But it can help you deal with the loss.

Go somewhere that knows about the lost repository, and mark it as dead:

git annex dead usbdrive

This retains the location tracking information for the repository, but avoids trying to access it, or list it as a location where files are present.

If you later found the drive, you could let git-annex know it's found like so:

git annex semitrusted usbdrive