Error when uploading

System.Web.HttpException: Session state can only be used when enableSessionState is set to true, either in a configuration file or in the Page directive. Please also make sure that System.Web.SessionStateModule or a custom session state module is included in the <configuration>\<system.web>\<httpModules> section in the application configuration.  
at System.Web.UI.Page.get_Session()
at ASP._controltemplates_gsasearcharea_ascx.checkPublicSearch(Object sender, EventArgs e)
at System.Web.UI.WebControls.CheckBox.OnCheckedChanged(EventArgs e)
at System.Web.UI.Page.RaiseChangedEvents()
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)