HTTP Error 404 - Not Found.

Hi,
I was working with my project nicely, after adding some new pages I faced with this error while trying to access pages from the menu bar.
only from menu bar I cant access to pages, even About,ContactUs. really I didnt apply any changes to these paths, I just add some new pages to a subdirectory.
what is wrong I cant understand.
I will be grateful if anyone helps me.
complete error message is this:

Server Error in '/asayesh' Application. HTTP Error 404 - Not Found. Version Information: ASP.NET Development Server 10.0.0.0