You can't join a game and participate in the middle of wave Combat (can only join in Building Phase).
You can only pray the monster gets unstuck by itself. You could be waiting a long time. In fact, it might never get unstuck and stay bugged forever.
In which case you have to quit. Sucks!
It's never happened to me before but my friends said it happened to them and they waited 10 whole minutes before quitting.
Pro solution: don't die in Hardcore