There are several reasons that may lead to the cancellation of a bet, but the most common one is when an event is canceled or an error occurs. In case of a single bet being canceled, the amount that was wagered will be fully refunded to the user.
If the game you bet on is part of a multiple bet and it gets canceled, the odds for that game are voided, and the multiple ticket is recalculated with the odds from the remaining games.