I've sorted out the problem and it turned out to be much easier to fix than I thought. Hopefully somebody out there who also knows as little about plumbing as I do can be saved some time and money by reading this.
One day in February, the shower lines seemed to have frozen, as no water was coming from the faucet. I ran the hot water on full, but it only slowly trickled. Finally, it was hot water dripping slowly from the tap, but I couldn't understand why it was only slowly dripping. I assumed something inside the faucet was distorted and jammed from the freezing. Then the cold line started dripping in the wall, so I shut off only the cold water line. We have pex pipes (plastic) that connect to a short piece of copper that then connects to the faucet. I read that pex would not likely split from freezing so assumed it was the copper, but it was in fact the pex pipes that split - so never assume. I had searched for a pex crimper a few years ago and heard that they sell for about $200, and that is true, but they also sell cheaper ones now. But even better, I discovered new fittings that slides onto pex or copper interchangably - no special tools required. That was unbelievably easy. So I fixed the cold water line and turned it back on. As soon as I did that, the hot water returned to full pressure. This must be a result of the balancing valve/spool value, which regulates the distribution of hot and cold, so you don't get scalded. Somebody can correct me if that's not quite right. Anyway, I didn't know such things existed inside faucets, so I had no idea that the hot water could come back once the cold line was turned on. I should have mentioned that I had the cold line off in my original post, but didn't realize the significance.
Thanks to jadnashua for the replies and the others who helped on this related post:
https://terrylove.com/forums/showth...ly-will-pex-pipe-melt-near-a-soldered-fitting.