If you install IIS after .NET framework or if you removed and reinstalled IIS on a server that has .NET framework, then your ASP.NET pages will not work.
This is because you are missing the IIS mappings associating the ASP.NET file extensions.
To fix IIS mappings for ASP.NET, follow these steps:
1. Click Start -> run -> cmd - ENTER
2. At the command prompt, type the following, and then press ENTER:
In this path, version represents the version number of the .NET Framework that you installed on your server. You need to replace with the actual version number when you type the command.
3. Register the Aspnet_isapi.dll by clicking start -> run
4. In the Open text box, type "regsvr32 %windir%\Microsoft.NET\Framework\version\aspnet_isapi.dll" and then press ENTER