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Assuming you have a standard toilet the first thing to do is take the top of your toilet tank off. You will either have a float on a metal arm that is attached to the fill valve or the fill valve it's self will have a small float on it. The fill valve is what lets water into the tank.
If you have a float on a metal arm do the following. Lift straight up on the float slightly then push in a downward direction on the float towards the bottom of the tank.
Did you get any water entering the tank?
If you have the type fill valve that has a float on it do the following. Take your hand and slide the float up a little and then downward towards the bottom of the tank.
Did you get any water entering the tank when you did this?
If you did you will need to replace the fill valve, it is sticking in the off position.
If you did not get any water going into the tank when you did the above, do the following.
First, you will need to verify that you have water pressure to the toilet fill valve.
You should have a cut off valve behind the toilet near the floor. Check to see if this valve is open. If it is open, to check if you have water pressure you will need to do the following.
Cut the water supply valve off and then disconnect the supply line going to the bottom of the tank. Take a bucket of some sort and then cut the water supply valve back on. You should have plenty of water pressure after cutting the valve back on. If you have good pressure there, then it's back to replacing the fill valve again. The fill valve has mal-functioned and needs replacing.
If no pressure, your problem is in the water supply line. Check to see if supply line valve is opening, it may need replacing. If valve is OK you must go backwards from here to find the problem area. The problem could be a supply line valve is turned off somewhere else?