Displaying Alerts

1. Page.ClientScript.RegisterStartupScript(this.GetType(), "showSaveMessage", "<script language="'javascript'">alert('USER Deleted Sucessfully');</script>");

2. ScriptManager.RegisterStartupScript(this, this.GetType(), "@#$", "alert('No Items mapped with selected Party');", true);

Before or after this are you using any response.redirect..... 

Try ScriptManager.RegisterstartupScript.