Before the battery is completely depleted, the car will show several warnings on the dashboard.
If you keep driving and the battery reaches 0%, the electric motor will stop working and the car will stop.
In a traditional ICE vehicle, you could grab a jerrycan, go to the nearest petrol station and fill up your car. That’s impossible in a Tesla. So you need to call Tesla Roadside Assistance or a towing service to have the car towed to a nearby Supercharger or another place where it can be charged.
Note: Tesla battery packs always have some reserve capacity. But this capacity is not used to power the electric motor. It’s simply for safety reasons, because the battery can be permanently damaged if it’s drained until it’s completely dead.