Shower hot water not working, cold water only comes out?

I have a single handle shower valve. When i turn it to hot, only cold water comes out...at the same rate as when turned to cold. Hot water does work in the bathroom sink, as well as in the rest of the house. I have tried running the hot water in the sink at the same time as the shower, hoping it would... show more I have a single handle shower valve. When i turn it to hot, only cold water comes out...at the same rate as when turned to cold. Hot water does work in the bathroom sink, as well as in the rest of the house. I have tried running the hot water in the sink at the same time as the shower, hoping it would "prime" it. No luck there. I cannot see a brand name on the shower valve, but I believe it to be 40 years old, I highly doubt it has been changed out since house was built in 72. It's never been a problem before, but admittedly it's been probably 2 years since it was used.
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