My toilet cister is not filling with water at all. I have looked inside and the black fitting that is connect to the water pipe is not letting any water in.

If the "black fitting" is the ballcock on the left I have a solution for you. Let me know Tom

There is a float on the black pipe. Is it in the up or down position? This unit is called a Flomaster.

If in down position, shut off water and remove braided hose from bottom of toilet. Aim into bowl or pot and turn water on for a couple of sec. If u get water, replace flomaster.

If no water u have blockage in shut off valve.

Chuck

Answer Chucks questions and I have another takeon this, The little hole in the seal can swell shut cutting off the flow of water no matter where the float can is.
The solution would be to replace the seal,(see image).
Here's how to do it.
So you have a FluidMaster ball cock. This is a common problem with these ballcocks. They have a small washer with a hole,(see image) in the center that swells shut in time preventing water from entering the tank.
Purchase a FluidMaster Seal#242 from your local big box store. To replace it you will have to shut the water off to the tank and take the tank lid off. Twist the top of the ballcock off counterclockwise, (see image) and pull up. Pry out the seal. Now place a pan over the open ballcock and turn the water on fro a few seconds to flush out the supply, Now replace the seal. Reassemble, turn the water back on and give it a flush. Better now? Regards, Tom