asp.net unhandled exception

The client disconnected.System.Web   at System.Web.UI.ViewStateException.ThrowError(Exception inner, String persistedState, String errorPageMessage, Boolean macValidationError)     at System.Web.UI.ClientScriptManager.EnsureEventValidationFieldLoaded()     at System.Web.UI.ClientScriptManager.ValidateEvent(String uniqueId, String argument)     at System.Web.UI.Control.ValidateEvent(String uniqueID, String eventArgument)     at System.Web.UI.WebControls.DropDownList.LoadPostData(String postDataKey, NameValueCollection postCollection)     at System.Web.UI.WebControls.DropDownList.System.Web.UI.IPostBackDataHandler.LoadPostData(String postDataKey, NameValueCollection postCollection)     at System.Web.UI.Page.ProcessPostData(NameValueCollection postData, Boolean fBeforeLoad)     at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)