Whoops! As a side effect of the menu controls becoming rebind-able in update 43, anyone who had modified controls will probably find that the in-game menu doesn't open when they press ESC or M any more.
You can either go to the edit controls menu and add a key for the menu (unfortunately, I've also realised that you won't be able to enter ESC for this, as it currently just cancels the key input), or just reset the controls to default, which is a function available in the edit controls menu.
Sorry for the inconvenience!
I've got just over two more weeks to pull this kind of nonsense after which I will try to avoid this kind of thing so far as possible...
-Jon