If your toilet is the old-fashioned sort with a toilet ball that floats in the tank to turn off the flow of the water, these can develop holes. If a hole develops in the toilet float ball, the ball no longer floats and your toilet will keep running. If this happens to you, here's a quick fix until you can replace the mechanism:

Remove the toilet float ball from the arm. It should just screw off. Empty out all of the water and completely dry off the ball. Lift up the flap at the bottom of the toilet tank and keep it up so that the tank is completely emptied. While the tank is empty, screw the toilet ball back on to the arm. Then put a plastic bag around the ball and tape the bag closed. Make you sure you put the tape at the very top where it will not get touched by the water. This repair should last for a couple of weeks at least which will buy you a little time for you to replace the ball or the complete mechanism.