You have a good few minuets to reconnect. So when you do disconnect press refresh to reload the page and take you to the same battle room. If you fail to reconnect you loose due to battle timer/inactivity unless your opponent also fails to complete the 2 minuets or so of waiting. As soon as the timer hits 0 while your opponent is in the lobby still you loose and the ladder updates so make sure you refresh.
It can be glitchy sometimes not allowing you to pick a move once reconnected.