When the toilet overflows, it’s embarrassing and panic-inducing but it’s a crisis that you can stop. Many toilets are designed so that the bowl can hold all the water in the tank but this is not always the case. You can plunge or snake away the clog later – the most important thing is to stop the toilet from overflowing.
Take the lid off the tank. Place it carefully across the toilet seat in the meantime. You don’t want to break it or have it slip and fall if leaned against the wall. This is the safest place for it. Next, reach inside the tank – don’t worry, that’s fresh water in there, so it’s sanitary. Push down the rubber flapper valve that empties the water from the tank into the bowl. It should be located in the center of the bottom of the tank. This will stop the flow of water from the tank into the bowl.
Then lift the ballcock mechanism – that is, the floating lever that signals the tank to fill. This will stop the flow of water into the tank (in the upright position, it thinks the tank is full and stops filling it with water). If the water in the bowl drains as you’re doing this you can release the float when it is empty. The tank will refill but the bowl won’t overflow.
But, if the water level in the bowl doesn’t drop, you’ll need to shut off the water supply. You’ll probably need help, as it’s hard to hold the ballcock mechanism and turn off the supply valve behind the toilet on the floor.
If the water level is already high then flushing the toilet again will surely make it overflow. Stop the flow of water into the bowl, wait for it to drain and shut off the water supply if necessary. Then you’ll need to plunge or use a snake to remove the clog, opening up the pipes for proper flow once more.