My mom has no water in her bathtub. The faucet is a single handle knob, that you pull out to let the water run, and turn either left or right for hot or cold. I do not know the brand of the faucet - it's the original from 40+ years. She has water to the rest of the house (sinks, toilet, etc.), so we know its not the water supply. Any help?

Show us a picture of the valve.

I don't have a picture of the valve yet - I haven't tried to disassemble it yet. Also - just found this on your site :

https://www.askmehelpdesk.com/plumbing/no-water-coming-out-bath-tub-faucet-362706.html

This seems to be exactly what is happening to my mom! I even think that her faucet is a Moen (my memory is fuzzy on this, but it seems to ring true), so it ties into that link perfectly! I am going to work on it tomorrow and update my progress.

If it is a Moen valve. Turn off water, remove center emblem of valve handle with sharp pointed tool. Remove screw that holds handle on. Remove handle, remove the chrome or brass cover sleeve. Simply pull out with pliers. Remove horse shoe clip that holds cartridge in valve. Reinstall handle, use handle to pull cartridge out.

Turn water on briefly to flush any debris in valve.

Reinstall cartridge. When reinstalling cartridge install it with the stem pulled out (open position). This will allow air trapped in the valve to escape so that the cartridge can seat properly.