The number one cause of a toilet tank not filling up is a faulty fill valve. Over time, the inner workings of a toilet tank wear out and need replaced. When the fill valve begins to wear out, it can stick in a closed position, preventing the tank from filling.
Replacing a fill valve is generally a simple process and requires only a few common hand tools. After purchasing a new fill valve and ensuring that the water supply to the tank is turned off, the toilet should be flushed to remove any water that may remain in the tank.
Fill valves connect to the toilet via a small hole in the bottom of the tank itself. This is also where the water supply comes into the tank, often via a flexible hose. The hose should be unscrewed from the bottom of the toilet tank after ensuring that it is turned off. Generally, a plastic nut on the outside bottom secures the main valve assembly. This nut can be removed with a pair of pliers, channel locks or a pipe wrench. Once it is removed, the assembly can be lifted out of the tank and the new one placed back in.