if you are using updatepanel try below code
ScriptManager.RegisterClientScriptBlock(this.UpdatePanel1, this.UpdatePanel1.GetType(), "Msg1", "alert('Saved');", true);
or
Page.RegisterStartupScript("Alert", "<script>alert('Successfully Registered and now u can login')</script>");
Page.ClientScript.RegisterStartupScript(this.GetType(), "Alert", "alert('Upadted')", true);