I am using Git for a project.
Up to yesterday night, the repository was up and running and I shut my computer down by the end of the day as I always do.
This morning, when trying to run
fatal: not a git repository (or any of the parent directories): .git
Obligatory question: Do you have a backup?
If not: bad.
If yes: Restore the backup of the repository to a different directory, check out the latest branch / commit you know you were working on, if there's a stash in that backup as well, commit it to a new branch. Then overwrite the source files in that branch with the source files of your damaged repository. This gives you the difference of the last undamaged, backed up state and whatever you did in the meantime. You can stash it as well.
For this very reason I normally commit early, often and push to an external repository as backup.