I found a bald spot on the top of my head, it's a little bigger than quarter size. What's wrong?

Im 26 and have always had thick, healthy hair! I fix my hair every day and the bald spot was just there like over night! What could be the cause of this?I found a bald spot on the top of my head, it's a little bigger than quarter size. What's wrong?
You should see your dr. There are a few reasons why you may have hair loss, but I would advise you to see your Dr to rule out anything serious.