The TextBox.TextChanged event is raised when the content of the text box changes between posts to the server. The event is only raised if the text is changed by the user; the event is not raised if the text is changed programmatically.
A System.Web.UI.WebControls.TextBox control must persist some values between posts to the server for this event to work correctly. Be sure that view state is enabled for this control.
For more information about handling events, see Consuming Events.