The Toilet Doctor is in.
For such a simple mechanism, there sure are a lot of things that can go wrong with a toilet.
The “hope they never happen” problems are septic or sewer backups, clogs, or a cracked or broken tank or bowl or a leak from under the toilet.
Minor toilet problems generally have the same symptom, running water, so the diagnosis may take more time than the repair. The problem is sometimes so subtle you can’t even be sure it is happening. If you are on private water, the pump might kick on when it shouldn’t or the running sound stops almost as soon as it starts. In that case, put a dye tablet (town water departments often them for free) or drops of food coloring in the tank then try not to use that toilet for a few hours. If the water in the bowl changes color, even a little, water is leaking in. Now you have to find the source.
Remove the tank lid and take a look.
If you are not sure how a toilet works, flush a time or two and watch. If the water is running but the tank is not filling completely or at all the rubber flapper or tank ball that covers the outlet between the tank and the bowl is not functioning correctly. When the toilet is flushed that flapper is lifted by a chain or the ball by a wire attached to the flush handle and should drop back into place when the handle is released. If the flap/ball is misaligned so it doesn’t fit squarely over the drain the water will run constantly or intermittently into the bowl. This can be caused by lime deposits on the flap or the rim of the outflow tube, or perhaps the rubber has stiffened or disintegrated with age. The flapper or the chain may be caught on something or the wire is bent or the flapper wedged on its hinge.
Misaligned Float Problems
The other cause of running water is an improperly aligned float, a big plastic ball lifted by a horizontal a rod or a hollow donut shaped piece of plastic that rises and falls on a vertical rod, shutting off the water when it reaches its apex. If the water fails to shut off, it will rise over the top of an overflow tube in the center of the tank and run into the bowl. It should be quickly apparent if the problem is with the flap or the float although sometimes it is a combination of the two.