Could not load file or assembly 'AjaxControlToolkit' or one of its dependencies

It misses the AjaxControlToolkit.dll in the bin directory after you deploy the site.

Solution:
1. Get AjaxControlToolkit via http://ajaxcontroltoolkit.codeplex.com/releases/view/43475
2. Add AjaxControlToolkit.dll to your Reference folder(Right Click on Reference Folder  > Add Reference > browse  AjaxControlToolkit.dll).
3. Set Copy Local to True
4. Publish your site.