I think the health is stored in your player file in the world folder. You could use some plugin to set your health back to 20 (which is 10 hearts), or try deleting your player files (Keep backups though! This will also delete your position and inventory)
Thank you, I will try this imediatly! Thank you this worked! EDIT by Moderator: merged posts, please use the edit button instead of double posting.
quvin4 Never actually did it myself, but I think it's a box when you edit the first post. Correct me if I'm wrong!