Is she a breeding mare? Pregnant? Foaled before?
Yes an umbilical hernia is possible, some horses have them and they're fine, just a little bump, and they can be left like that and be fine. However, if this is new I'd get it checked. Sometimes the vet can just poke it back in with his finger and it will magically close up on its own. Sometimes it requires further investigation. It is usually caused by a torn muscle and the stomach intestines poke through, or fat layers opening etc etc........actually it's a big subject, I've just realized this is going to take more typing than I'm able to do right now, but definitely get your vet to look at it.