Hi,
If the order is missing, then it's probably that it was deleted by one of the persons with access to the backend.
It could also be that there is a mass action wrongly configured and which deleted the order (menu System>Mass actions).
You can actually use a mass action to search for the order with the "display the results" action and a filter on the order column on which you want to filter the orders.
It's potentially possible that the order_type of the order was modified (by a mass action, someone with access to the database, or a third party extension) resulting in the order not appearing on the orders listing of the backend. But in that case, it will still appear with a mass action if you search for it.