First, you need to stop the water flow to the toilet. If you are able, shut off the toilet at the supply stop. Make sure the tank is sealed from the bowl. Then, attempt to plunge the toilet with a plunger. If that doesn’t work, see if you have a sewer line cleanout in the yard. Open the cleanout to release water outside into your yard instead of into your house. Beware, there may be head pressure in the drain line. Then, call a plumber to auger out the lines.