How to stop a running toilet...

Assure that the float ball isn’t caught on anything else, it can be caught
on the chain or stuck in some other way, if so, just free it, and make sure
it not rubbing up against the side of the tank, if so, move it toward the
middle. Also, make sure the float ball doesn’t have a leak or is full of
water; the extra weight could stop it from lifting freely. If the float
ball valve and the fixtures are covered in corrosion, it could also be a
reason for preventing the toilet from working properly. You can get you
lime away or other chemical and try to get the corrosion off, or simply buy
a new float mechanism.

Try cleaning or replacing the flap or flush valve, if the toilet stops
filling then starts again, or water runs into the bowl constantly, you may
have a slow leak from the tank into the bowl. In order to know this for
sure, you might want to try placing a couple of dye tablets in the tank. If
after an hour without flushing, there is dye in the bowl, you have a leak
and there’s a small amount of water running into the bowl.

Usually small leaks are caused by a rundown flapper try cleaning or
replacing the flapper if, after pushing it into pace, the running sound
stops. You can also try adjusting the small rubber fill tube leading from
the valve to the overflow tube and sometimes the valve itself can act as a
siphon. In that case, adjust the valve height or tube height up, or adjust
the water level down. The water pressure is another source you might want
to look at, try adjusting the flow of water at the shutoff valve and see if
less water pressure stops it from running.

(This is not an exhaustive book on the issue, just some possible steps that
may help)!