How do I undo a checkout in git?

Try this first:

git checkout master

(If you're on a different branch than master (or main), use the branch name there instead.)

If that doesn't work, try...

For a single file:

git checkout HEAD /path/to/file

For the entire repository working copy:

git reset --hard HEAD

And if that doesn't work, then you can look in the reflog to find your old head SHA and reset to that:

git reflog
git reset --hard <sha from reflog>

HEAD is a name that always points to the latest commit in your current branch.

To undo git checkout do git checkout -, similarly to cd and cd - in shell.