How can I stop or reset redstone contraptions based on player deaths?

Add a dummy scoreboard.

/scoreboard objectives add find dummy

Summon an armor stand as a game marker.

/summon armor_stand ~ ~ ~ {CustomName:"\"finder\""}

Then, at the start of the game, have this command run:

execute as @a[targetallthehiders] run scoreboard players add @e[type=armor_stand,name=finder] find 1

This will give the armor stand a score that is equal to the number of players.

Every time a hider dies, remove one point from the armor stand. When everyone is dead the score will be 0, and you can target the armor stand like [type=armor_stand,name=finder,scores={find=0}].

Does this help?