Exactly that happened at Paris-Las Vegas, which blacked out at 9:45 a.m., Thursday November 3, 2016.
In most cases of electrical failure, auxiliary power will kick in, giving slot players time to cash out their machine credits (taking TITO vouchers from the machines and getting cash from a kiosk or the cage) and table-game players time to cash in their chips from the cage. If it's a large amount of chips, it's advisable to immediately cash them in; cashiers have to verify where the chips come from, so a time lag might make that difficult, and casinos are under no obligation to accept cheques that could be under contention.
In the Paris case, however, construction workers drilling in the bowels of Paris accidentally sliced through all the power cables, including the auxiliary system, leaving the property, basically, helpless in the dark.
In a similar situation, slot players are advised to wait by their machines for a slot attendant or employee from guest services to arrive and establish how many credits they're due, then obtain a voucher that validates winnings. This might be tough to pin down exactly, since the machine, like the lights, are dark. But it will establish a ballpark figure that can later be double-checked once power is restored: The machines have a fail-safe that records the state of play at the time it was interrupted.
In theory, once the slots power back up, play should not resume until the player returns to the machine. If he or she doesn't, the casino can take the information it gleaned from the device and send the player his or her money. (And in case anyone is tempted to make a false claim, surveillance footage is checked to verify that the players were on the machine they say they were.)
Of course, all this advice pertains to a relatively minor emergency like a power outage. If, God forbid, the fire alarms go off, forget your chips or your slot credits and get the heck out of there. You'll thank us later.