Lift the cistern lid and you should see the water level much higher than normal. You should see the water flowing out the overflow pipe. (or with a modern siphon into the pan) You will need to change the valve. Turn off the water. And flush the toilet.

To fix an overflowing toilet, wait for the water level in the bowl to drop a little or drain out some of the water using a cup. Use a flange plunger to unclog the toilet. If the plunger does not clear the clog, snake the toilet drain line using an auger. An overflowing toilet may fix itself, but that is not always true.

There are typically four main reasons for a toilet overflowing. The drain line may be clogged, the plumbing vents are blocked, the main sewer line for the home is plugged, or, if you have a septic system, the septic tank may be full or obstructed, causing the waste water to backflow into the home.