404 just means your web server cannot find the requested resource. Most probably the path to the aspx page is incorrect, you need to check that. In general, for a normal ASP.NET app:
http://server/directories/page.aspx
Where
server is the name or IP address of the server, most likey
localhost. If you are running with ASP.Net Development Server (a balloon will pop up, this will give you a port number (e.g. 4907) in which case server would be
localhost:4907. This is randomly assigned so I can't tell you what it is.
Page is the name of the page you want probably
default
Directories follow the directory structure of the app. If your document is in root you can just leave this blank and remove and double forward slashes.
Your required url is most likely to be:
http://localhost:4907/Default.aspx
Where
4907 needs to be replaced with your port number.
Without more information it is hard to give a specific answer. One thing you can do is to right-click the web project and set that as the startup project, then right-click the page you want to display and set it as the default page. Running the application should then work.
One final thing, I've had problems with Visual Studio not serving up pages in the past. After I eliminated the possibility of an incorrect URL as a reason, a machine re-boot fixed the problem.
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