Scripts for beginners.

function PlayerLooseCheck(pl)
while 1 do
if GetPlayerState(pl)==PLAYER_LOST then
Trigger(...trigger reset)
end
sleep(10)
end
end;

startThread(PlayerLooseCheck, PLAYER_PLAYERNUMBER)

You can do it like this, for example. Alternatively, you can attach a trigger to the event of a player losing a hero, and use that to determine if the player has lost – this would put less load on the game, but I'm not sure if this method would work.