Our sink's hot water won't shut off. We have tried what we thought was the shut off valve, located under the sink with no success. Any suggestions on how to shut if off.

If there is a valve under the sink and it is hot after the hot water has been running then that is the cut off for the sink faucet hot water. Did you mean that you could not turn it or that you turned it and it did not cut the water off? If it did not turn off the washer is probably gone from inside the cutoff, and you will have to find another cutoff for the hot water, Have you looked on top of the water heater to see if there is a cutoff there. It should be on the pipe that enters the water heater on the right side.