My shower has two handles, one for hot and one for cold water. This morning when I turned the hot water knob off after my shower it worked fine at first but then the handle started to feel like it was turning really loosely and it never came to a hard stop (it kept spinning in the same direction as long as I turned it). The hot water turned off, though. I thought this was weird so I tried to turn the hot water back on but now it just spins freely and no water comes out at all. The cold water knob still works fine, however. It seems like something is wrong with the knob but I'm not a plumber, so here I am. Thanks!

A brand and model for that valve would help us determine if you have a failed stem and seat or a bad cartridge. I am thinking stem. Sut off the main supply or a stop that controls this valve. Remove the packing nut and see if there is a stem that can be backed out with a wrench. tub and shower repairs two handle - YouTube