ASP.Net AJAX Control Toolkit - ExtenderControlBase.cs does not exist

When i am running the existing website it is throwing the following exception

The file 'd:\hg\act\Server\AjaxControlToolkit\ExtenderBase\ExtenderControlBase.cs' does not exist.

I have ajaxcontroltoolkit.dll in my bin folder.I have tried by removing and reinstalling it.but it is still showing same exception.

what could be the cause and how to solve it??

Thank in advance..