I am not really sure of a reason that will make the smart tag disappear. It could well be some glitch or one-time problem with visual studio. In this forum thread
a suggestion to clean the temporary asp.net files seems to have helped. In addition I would suggest that you remove the references to Telerik controls, close Visual Studio (all instances) and then re-open it and add the references again.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed