Basically this https://en.m.wikipedia.org/wiki/Lokomotiv_Yaroslavl_plane_crash happens to any major sports team on their run to the championship. What happens?
It happened a few times. A more notable example would be accident where Manchaster United's team got killed in the 50s.
It would take some research into league rules for the major sports, MLB, NBA, NFL, NHL. I am sure there are some obscure rules on how to handle the situation involving the total loss of a playoff team.