In the tank, if you have the type of fill valve that has a round float that rides up and down on a post, they have a diaphragm in them with a tiny hole that can get plugged. Very often when you shut a water line off, sediment or rust breaks off the valve and floats down the line and plugs something. Shut the water off, hold the shaft of the valve so it doesn't break and twist the top of the assembly 1/8 turn counter clockwise. The washer is where the two pieces come apart. Take it out and turn the water on just a little bit. Too much pressure and you will hit the ceiling with water. Let it run a bit and that should flush out any particles. Put it back together and it should work. Make sure to put the washer in the right way down.
That device is called the refill tube. This allows water to go down the overflow tube to refill the water closet bowl while the tank is filling.
Refill valve is a very important component in your toilet since it fills the tank of the toilet after every flush. This valve uses a float to turn the water on and off. This is a plumber related work, only a professional plumber can repair it. alliedallcityinc.com
Assuming you have a regular tank toilet with a ball float and rubber flap and that the water is turned on and the tank is filling, the most common cause of a toilet not flushing is a broken or detached chain that runs from the handle lever to te rubber flap that keeps the water in the tank.
If the holding tank is full you can flush it just once. Further flushes will have to wait until the holding tank is again full. If the tank is empty- no. If you have a way to refill the holding tank with water again, you can flush one more time for every time you refill it. When our water was going to be off for a time, I pre-filled the bathtub with water to use for this purpose.
Toilet tank
First check to see if the valve for the water supply is turned on. If that isn't the problem you may need to replace the tank water valve, they do wear out and are not that expensive or difficult to replace.
There should be no reason the toilet would overflow. The water being turned off should have no effect on it. As long as the drain is clear even if the shut off valve in the tank didn't work, the water will just go down the drain.
Each flush of a toilet uses the same amount of water.
A vacuum breaker is a device that prevents water in a toilet cistern or water tank, from syphoning back into the toilet cistern or water tank.
That is a fill valve and is part of the ballcock system. When someone pulls the lever, that pulls the chain and lifts the flush valve flapper and empties the tank into the bowl. That causes the float to drop, opening the fill valve and allowing water from the supply to enter and refill the tank.
Check the flapper and/or the ball cock assembly. Either or both probably need to be replaced.
seal to stop water from runing into tank located between tank and bowk