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Much more likely the cartridge has seized up and needs to be replaced. But you'll need to remove the handle to replaced the cartridge. I need a picture of the handle showing the set screw to answer that question. You can post a picture here by clicking on ADD FILES next to the SEND button
Ok, have you tried backing the allen set screw out?
It's usually 1/8" or 5/32". No way I can know for sure with this old style. I have a set and try each size until I find one that fits
Yes it's not a metric size.
It's not uncommon for these valves to seize up. I have no idea where you live and what the weather conditions are there. You made no mention of freezing conditions and all I know is what you tell me.