Most times a running toilet is the result of the flapper valve not sealing completely after a flush. If the flapper valve is worn it may not be containing the water in the tank anymore, resulting in water trickling into the bowl on a non-stop basis. When this occurs its usually time to replace the flapper, which is a relatively easy maintenance task. You can buy a replacement flapper valve at any home improvement store. When you get home, turn the water supply off to the toilet, remove the old valve and replace with the new valve. You would be surprised how much water is wasted when this occurs - up to 200 to 300 gallons per day.