I was able to build and run my web sites.But yesterday something happened,now i can build but can not run the appllication.
I mean it opens internet explorer with an error mesage that says the page could not be found or you are not connected to the
internet.But when i click the small icon in the task bar which is asp development server ,and clicik the link in that window,
it runs correctly.Please help,its a terrible thing.

internet explorer can not display the web page.

Hi,

Based on your description, I know your application is File-Based type.

If we create a IIS-Based application, does it have the same issue? We need to make sure that IE works fine. 

Did you install Antivirus Software recently, which can affect ASP.NET Development Server. For more information, see http://forums.asp.net/p/1235447/2242010.aspx

I hope this helps.