Suddenly an ASP.NET website was down...
I didn't do anything new to the web app there. Upon inspection I got message saying "applicationhost.config Is Not A Well Formed XML".
This indicates that file in "C:\Windows\system32\inetsrv\config\applicationHost.config" was corrupted. Normally you can find its backup inside "C:\inetpub\history\" folder. Don't forget to restart IIS after that.
In our case, this corruption was caused by hard disk space.
Thanks to this StackOverflow post.