I was getting following error message
Framework64\v4.0.30319\Config\web.config file is not well formed while opening IIS manager.
The solution
Solution was very simple, because of some installation my web.config file was modified and one closing tag was missing.
I copied pasted the xml content to online XML validator website and found out the error that my <assemblies> tag didn't had its closing tag. I added that and everything worked fine.
Hope this helps someone!
Thanks