Broken theme

I was checking the code for the sidebar in the Theme Editor when I clicked it it took me to the index of my site. Then when I went back to the Themes it said that the active theme I was using (which I didn’t change recently) is broken.
What’s going on? I didn’t change anything, it just says it’s broken now.

EDIT: This was all caused by a Theme Switcher plugin I had activated. I deactivated it and everything’s back now, phew.