Sys == undefined using AjaxControlTools, please Help me!!!!!!

hello, for one week, there is, in my site (localhost) the following error

"Error en tiempo de ejecución de Microsoft JScript: Error al cargar el marco de trabajo de cliente ASP.NET Ajax.", in english "Runtime Error in Microsoft JScript: Error loading framework ASP.NET Ajax client."

is a mistake typeof (Sys) === 'undefined'

I have read on forums, several comments, but none gives an effective solution Can anyone help?, Until one week ago, I had no problem using the AjaxControlTools 4

Thanks in advance