Fault one — it's overflowing. Stop it in ten seconds: lift the tank lid and push the flapper down by hand. That's the fastest intervention available and it stops water leaving the tank immediately. Then close the angle stop — the small valve on the supply line behind the toilet — turning clockwise. What not to do: don't flush again to see whether it's cleared. That's the instinct, and it delivers another tank of water onto your floor. Where the fix is: downstream of the bowl, not in the tank. It's a blockage in the trapway or beyond, and the tank parts had nothing to do with it. Cost band: a clearing, and inexpensive if it's the fixture. More if it turns out to be the line — see below.