Hey Tom 
Sorry to hear about what happened. It might be worth noting the below in future, I know that in the moment this might not be the first thing to help about.
Unfortunately, this is a waiting game, I’m sure Monzo will do their best to get to the bottom and resolve it, but it’ll take time.
All you can do is wait, as hard as that must be,
Good luck in getting it resolved and please update us here if you can