First time poster... Everything was working fine until yesterday, when the city shut off the water main on my street for a few hours. When it came back on, I was suddenly not getting cold water in one of the showers in my house. Everything else is working fine, including the sink in the same bathroom (hot & cold, pressure, no problems). This is in the upstairs bathroom. Hot works fine, it's just the cold. It's a single tap. All the way left (about 10:00) is full cold, all the way right (about 2:00) is full hot.

I've left it running on full cold, hoping it was just air in the pipes from the shut-off that needed to flow through, but after quite a while, all I got was a tiny trickle. I've read similar posts here that indicate it might be the pressure balancing valve, but I'm suspicious of a couple things: 1) it was working totally fine before the main shut-off occurred, and 2) this shower is used at least twice/day, so it's not like it should have been stuck from low usage.

I took apart what I could, but I'm not 100% sure what brand the valve is. It was in the house when I bought it. It may be Delta, but again, not sure, and I haven't seen any pics posted in the forums that look like it. So, here's a picture of it.

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I like to think I'm handy (but ask my wife to be sure), and I can follow instructions (again...), but I'm certainly no plumber. I'm not sure where to go from here. I don't know how to check for a pressure-balancing valve, any kind of cartridge, or what that even would look like. There also doesn't seem to be any more obvious ways to take anything apart without breaking something. Is there anything I can do here myself before calling a pro?

Thanks very much.