The answer to this question is dependent on when your bet was placed and whether the runner nominated for the race.
Bet was placed Pre-Nomination and horse did not nominate
In this case, bets on runners which did not nominate for the race are voided (refunded). So, for a multi bet, the particular leg will be voided and the remainder of the multi would stand and pay out accordingly.
Bet was placed Pre-Nomination and horse nominated
If your selection nominated for the race and has since been scratched, unfortunately this will be settled as a loss. So, for a multi bet, unfortunately the bet itself will be a loss.
Bet was placed Post-Nomination
In this case, bets on scratched runners will be settled as a loss because the markets after nominations are 'All In'. So, for a multi bet, the entire bet itself will unfortunately be resulted as a loss.