In our secondary bathroom, our shower/tub has a single handle knob and when turned to HOT no hot water comes out at all and it just shuts off the water (it drips just a little bit when on full HOT, and that water is also NOT HOT). It is a condo unit and it used to work just fine and this particular shower has seen very little and infrequent use. All other faucets work perfectly fine in the condo.

What do you think the problem is and how can I fix it myself? Thank you for any and all help!