I have found the problem and solution too.
The problem was that I was using re-used dll file "AjaxControlToolkit.dll". It means I was
clicking in the menu Website->Add a reference
Then I was selecting the AjaxControlToolkit.dll
file from the Bin folder of downloaded projects. And thus, it was not working in my website.
I downloaded new AjaxControlToolkit.dll(only controls as recommended by the providers) for framework 3.5 from here
and used it as normally we do and it worked.