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 semitrust usbdrive