Server Application Unavailable

What is happening? Out of a sudden, when accessing asp.net website, this error message is showing up instead of a normal webpage.

Oh my god... Checked on the IIS, every setting is correct, verify file is there, permission is correct as well.

I even tried setting Everyone's permission to Full control but this is not solving the problem.

So my next step?
Re-registering aspnet_regiis.exe -i
(c:\windows\microsoft.net\framework\v2.0.50727\aspnet_regiis -i )

And Voila, everything works back to normal. YAY!!!

If the above still couldn't fix your problem, you can try the solution in this link, http://blog.crowe.co.nz/archive/2005/08/30/242.aspx

Cheers, :)