My bathtub will not turn off one knob handle

What's the brand name? How old is it? Without much information I would say you have some mineral build up that broke loose and is now blocking your valve from closing. Try this. Shut the water off and open up the valve. Remove the cartridge and check for trash. Now turn the hot and cold water back on to flush out the supplies. Reassemble and test. Let me know if that works for you. Good luck, Tom