So, I couldn't get to the default.aspx page as well. Went the reinstall route. Now I'm having numerous other problems. To start I cannot see any of my previous applications in Builder (I did .zip them, thankfully), yet I can browse to them in IIS. Do I have to somehow reregister the App folder with Builder? Seems like they only work as localhost now.
Also, I cannot connect to my ArcGIS Server services. Error states: "Invalid JSON primitive" - see attached.
Still, unsure how any IIS config changes occurred in the first place.