First things first, give the handle a good shake. It's possible that the problem is as simple as the handle being in the wrong spot sometimes. If turning the handle in different directions does not solve the problem, proceed to the next stage.
Check the flapper's credentials. The flapper, which is a rubber valve located at the bottom of the tank, is responsible for controlling the flow of water from the tank to the bowl. If the flapper is not properly sealed, the toilet will run, which will cause water to leak from the tank into the bowl. Additionally, the bowl will be filled with water. To examine the flapper, flush the toilet and watch it close to see whether it is working properly. If the closure is not working properly, the chain that connects the handle and the flapper may need to be readjusted. In the event that this does not rectify the problem, the flapper might need to be replaced.
Examine the fill valve carefully. The term "fill valve" refers to the device that controls the amount of water that enters the storage tank of a toilet. If the fill valve is not properly closing off, the toilet will continue to run continuously. In order to inspect the fill valve's end, look for a float ball or float cup on the end of the valve. It's possible that the fill valve isn't turning off correctly because the float isn't rising to the appropriate level. Check to see whether altering the float results in a resolution to the problem. If the float is already at its highest position and the toilet is still running, it is possible that the fill valve needs to be changed.
Check the interior of the bowl for any signs of a leak. If none of the aforementioned solutions work, it's possible that the toilet bowl is leaking. If there are any leaks in the tank, they can be found by placing a few drops of food colouring there for fifteen minutes. If there is food colouring in the bowl, there is a leak in the system that needs to be addressed.
Talk to a professional plumber. In the event that you are unable to handle the problem on your own, it is possible that you will need to seek additional assistance from a licenced plumber.