My guest tub dose not get hot water. I have hot water at the sink.
And the cold water at the tub has low pressure. Everything was working OK the other day.
Checked the valve at the washing machine,which is right next to the guest bath. That seems to be OK. What could the problem be?

Sounds like some mineral buildup from the heater got sucked up into the inlet ports of the cartridge and are now blocking the flow of hot water. The solution is to shut the water off, open up the valve and check and clean the hot water inlet ports of the cartridge. Before you reassemble turn the water back on to flush out the supplies. I would also recommend flushing out your heater on a monthly basis.
For long life and fewer troubles you should keep your heater clear of mineral build-up by flushing on a regular schedule. Let me show you how. 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. Don't forget to flush it out every month. Your heater will thank you for it. Hope this helps, Tom