Is it everywhere, or one fixture?
Run the hot tap in the kitchen and in a bathroom. If one is hot and the other isn't, the water heater is fine; the problem is at the fixture (a failed mixing cartridge in a shower valve is the usual holiday culprit, and a one-fixture call can wait until the day after). If nothing in the house has hot water, go to the heater.
Is it cold, or just not keeping up?
Lukewarm water that gets colder with each shower, on a morning when six people showered in a row, is a tank that's been emptied faster than it can recover. A standard tank heater needs roughly half an hour to an hour to recover a full tank in winter, longer with Chicago's 40°F incoming water. That's not a failure. Spread the showers out, run the dishwasher later, and it will catch up. If this happens every holiday, the tank is undersized for how you actually live, which is a conversation about sizing and tankless, not an emergency.
Water that's completely cold from every tap, with no recovery after an hour, is the heater.
Gas heater: look at the pilot and the controls
Is the pilot lit? Look through the little window at the bottom. If it's out, relighting once per the label on the tank is reasonable; if it won't stay lit, that's one of five specific parts and a plumber visit. Is the control knob set to "on" rather than "pilot" or "vacation"? Someone adjusting it before a trip and forgetting is a real thing. Is the gas on at the valve next to the heater (handle parallel to the pipe)? Does the furnace work? If the furnace is also out, the gas supply itself may be the problem, and that's Peoples Gas.
If you smell gas, stop, leave, and call Peoples Gas from outside: 866-556-6002.
Electric heater: check the breaker
Electric water heaters are on a dedicated double breaker. A tripped breaker, or one that's only half-tripped (the handle sits between on and off), is the most common cause. Reset it once. If it trips again, don't keep resetting it; a heating element has shorted or the thermostat has failed, and that's a repair. Also check for a reset button on the heater itself, behind the upper access panel, which trips when the water overheats. Pressing it once is fine; if it trips again, call.
Tankless unit: look at the display
Tankless heaters show an error code when they shut down. Note the code; it tells the plumber what happened before they arrive. The common holiday ones are flow and ignition errors from scale buildup, a frozen intake or condensate line in a cold snap, or the unit simply being asked for more than it can deliver with 40°F water and three showers running. Why tankless output drops in winter.
Water on the floor
Walk around the heater with a flashlight. Water at the base, a rusty puddle, or a steady drip from the discharge pipe on the side changes the conversation. A tank leaking from its seam isn't a repair and won't wait; shut off the cold water supply at the valve on top of the heater, set a gas control to "off" or kill the breaker on an electric unit, and call. Water from the relief valve's pipe is a pressure issue that also needs a plumber, but the tank isn't about to let go.
Deciding whether it's a holiday call
Call today if: the tank is leaking; there's any smell of gas; the house has no hot water at all and no obvious reset fixed it; or you have elderly guests, an infant, or someone medically fragile and no way to heat water otherwise.
It can wait until tomorrow if: one fixture is cold and the rest work; the heater is simply recovering; or you've got a workaround for the day.
Either way, the number is 833-758-6911. A live person answers on Christmas morning, and a union plumber from Plumbers Local 130 can be dispatched. Tell the dispatcher what you found in the checks above, whether it's gas, electric or tankless, and roughly how old the heater is. Get the price before anything is done. And if the heater is past ten years old and this is its second scare, plan the replacement for January rather than waiting for the third.