Per the documentation https://kite.trade/docs/connect/v3/postbacks/, if the order update for a given order is missed due to the unavailablility of the postback url ( due to some unknown reasons for say endpoint got down ) for a moment then, how could we retrieve that order update, since for that instant of time, have missed the update. In such cases, is there any other means to get the updates or have missed the update forever.
No, Postback order updates are sent once, so you won't know order status until you specifically make order's history API query.