There is only one reason why a toilet bowl can over flow and that is because it is clogged. You need to determine where the water is coming from that is continuing to cause the bowl to overflow before doing anything else. There are a couple of possible places the water overflowing the bowl can be coming from.
Start with the toilet that is overflowing, in many cases a simple clogged bowl can over flow if the toilet is an older one that uses larger volumes of water than today’s low consumption toilets. The low consumption toilets would have had to be flushed several times before the bowl would overflow or, a problem such as a leaking flapper or, leaking fill valve can cause the bowl to overflow. You should shut off the angle stop valve located on the wall under the left side of the toilet tank to stop water from coming into the tank. Once the overflowing has been controlled you can take the steps to unclog the toilet such as plunging or, auguring the toilet.
If the water coming into the tank of the overflowing toilet has been controlled and the water continues to overflow out of the toilet you are probably facing a clogged main line and the water is coming from other places in the building. This is more common with toilets on the lower floors of a building. If you are in a single family home get people in the rest of the building to stop letting any water go down the drain and the overflowing should stop. You then need to contact a plumber to clean the drain line. In multi family homes getting people to stop using water is more difficult and the landlord or, property manager should be consulted immediately. Another possibility is the line in the street may be clogged. If you have a toilet in the basement or, in some cases first floor that is overflowing and all possible uses of water in the building have been controlled yet the water continues to flow out of the toilet you may want to ask your sewer department to check their line for a problem. People facing this problem are often bewildered asking, “Where is this all coming from?” as their basement is flooding. If the water is not coming from you, where is it coming from? Prompt action is needed with city sewer line clogs as a lot of damage can result.