HTTP Error 503. The service is unavailable under simple ASP.NET 4.0 web site
I had to start the application pool that was set for my website and was automatically stopped on some error. (IIS (7.5 in my case)->Application Pools->Start stopped application pool.)
I've reinstalled .NET 64 - that helped.
P.S.
It seems like either some files in "C:/windows/Microsoft.net/Framework64/v4.0.30319/" folder or I removed them myself (VS sometimes complains on files in "ASP.NET Temporary files" and their deletion helps)... Probably I didn't pay attention that those folder not a temporary...
P.P.S.
In this case, why VS complained on files in "C:/windows/Microsoft.net/Framework64/v4.0.30319/" folder... ok, now it's hard to say.
Sounds like you forgot to enable .Net 4 extension on the IIS. Try finding and enabling it in ISAPI and CGI Restrictions
Based on the comment it might be that part of the framework was removed and in that case it might be wise to reinstall Framework 4 by first cleaning it up. Try this blog post which got a reference to a tool that automates cleaning up proccess http://blogs.msdn.com/b/astebner/archive/2008/08/28/8904493.aspx