Do not flush again. If the level is still rising, remove the tank lid if that is immediately practical and stop the incoming water.
Stop the refill
Lift the float or close the flapper if you recognize the parts. If water continues, turn the toilet’s shutoff valve clockwise. Do not force a seized valve until it breaks.
Protect the floor
Move rugs and place towels around the base. If contaminated toilet water has already escaped, wear gloves during cleanup and keep it from being tracked into other rooms.
Wait for the bowl level to drop before plunging
If the bowl is at the rim, adding a plunger displaces more water. Once there is room, use a flange-style toilet plunger with controlled strokes.
Do not turn testing into another overflow
Avoid repeated flushes just to check progress. A toilet auger is a reasonable next tool for someone familiar with it; improvised hard objects and aggressive drain chemicals can damage the fixture or plumbing.
Recognize a larger drain problem
If other fixtures gurgle, water rises in a tub or shower, or multiple toilets back up, the main building drain may be obstructed. Stop using water and call a plumber.
The damage-control order is stop inflow, protect the floor, let the level fall, then clear the obstruction.
Sources
- U.S. Environmental Protection Agency WaterSense — Fix a Leak Week — Household plumbing leaks, toilets, faucets, and water-saving maintenance.