Microsoft JScript runtime error: ASP.NET Ajax client-side framework failed to load

You suddenly might end up getting above message when trying to run your ASP.NET project. There various solutions given in forums and only after changing the web config as follows worked for me!

  <httpHandlers> 

 <remove verb="*" path="*.asmx"/>
 <add verb="*" path="*.asmx"  

validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
 <add verb="*" path="*_AppService.axd"  

validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
 <add verb="GET,HEAD" path="ScriptResource.axd"  

validate="false" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>

</httpHandlers>
 

reference 

http://forums.asp.net/t/1243449.aspx#2291128

cheers

Samitha