I have a single knob on my bath tub that controls the hot and cold water. The cold water
Will come out, but the hot water will not. There is hot water in the rest of the house and the same bathrooms sink.

It sounds like you have mineral build-up in your heater and it has been sucked up into the hotwater inlet ports of the cartridge. Turn the water off to the valve, open it up and check and clean the ports 0n the cartridge.
Also if you haven't flushed your heater yet let me show you how.
For long life and fewer troubles you should keep your heater clear of mineral build-up by flushing on a regular schedule. Attach a hose to the boiler drain at the bottom of the tank. With the pressure on, open the boiler drain and let it run until the water runs clear. You will see a spurt of red,(rust) followed by white grains,(lime or calcium carbonate). This shouldn't take more then a few minutes. Do this monthly to keep it clear. Now flush out your hot water lines on ALL fixtures. Now pull each aerator and clean the screens. Be sure you put them back togather the same way you took them out. Don't forget to flush it out every month. Your heater will thank you for it. Hope this helps, Tom