In attempting to remove a tag from a contact, i succeeded in deleting the whole tag for everyone. Is there a simple way to restore it?
Unfortunately not. Hopefully you have a backup of your database, and ideally a test site? If you load the backup on your test site, you should be able to search for people with the tag, select Export, and export only the Contact ID. Import the Contact ID list to your live site, and on the last page of the import, select Create a New Tag. You can enter the tag's name there. The import won't do anything other than create the tag, since it's only contact IDs.