ASP.NET Ajax client-side framework failed to load.

Hi,

I am using nopCommerce 1.5 and till now it was working perfectly fine.
But suddenly, the AJAX of the whole store stopped working. When I checked for the error in the ERROR CONSOLE, it gave the error as
              : "ASP.NET Ajax client-side framework failed to load."
              : "sys is not defined"

Now where ever Ajax ToolKit controls have been used they are not working, LIKE if AJAX TAB CONTAINER is used on a page then its not being rendered correctly, on inspecting the element its showing visibility as "hidden" for the TAB CONTAINER.
And all other toolkit controls have stopped working like the calendar

I have been using VISUAL STUDIO 2008 with SERVICE PACK 1

Kindly let me know what to do in order to solve this issue.

Regards.
Manpreet Singh