It’s happened in soccer a few times, most notably:
1949 the entire Torino team, save about 4 players, were wiped out in an airline crash. Torino at the time were the absolute dominant force in Italian football and well on their way to their fourth consecutive league title. A vast swathe of Italy’s best players were wiped out in moments. Torino were declared champions and the remaining four fixtures were played out by Torino’s youth team against their opponent’s youth teams.
In 1958 English champions Manchester United were decimated in another airline crash. 21 year-old Duncan Edwards, who despite his very short career, many recognize as one of the all-time greats in soccer, died shortly after as a result of his injuries. Though just about as many players survived as died, some survivors were left unable to play again and others took time to recover. One notable survivor was Bobby Charlton who is recognized as an all-time great and went onto win the European Cup with Manchester Utd and World Cup with England. Manchester Utd had to play out the rest of the season with a makeshift team including many youth and reserve team players. As a result their attempt to win the English league again collapsed and their attempt to dislodge Real Madrid from their dominance in the European Cup also failed.