Thank you! Very helpful. Can I just call a plumber to handle, or do I need to find out the model beforehand? I am not too handy so I would be reluctant to try to take this apart myself.

If you have a rotating, single handle controller (builders like Moen or Delta), you likely need to replace a cartridge in the handle. The cartridge has rubber seals that control the water to shower, and the rubber seals breakdown overtime and stop working. Usually, it means a leak, but it can also block the incoming supply. You can buy replacement cartridges at any hardware store, but you will need to know your brand/model. Sometimes, I need to remove and bring the old cartridge to the hardware store to match it.

Changing the cartridge is very easy. Look at the brand on your faucet handle and look at videos on youtube. For example, here is a decent video about changing a Moen cartridge.

https://www.youtube.com/watch?v=bEDxl0A2pl0

As mentioned in the video, stick a flashlight in the pipe to see if their is any debris when you are changing the cartridge.

My only comment is that I never have needed a cartridge puller (which is pretty expensive). I unscrew the handle, remove a clip that holds the cartridge in the handle, and then I re-screw the handle and pull out the cartridge using the handle. Another thing to note is that the cartridge can be flipped so that you get hot water from the left or right. Rotate the cartridge 180 degrees if the setting seems reversed (you get hot when the handle is turned to could).

OP here. Accidentally posted too soon. Second floor shower isn't getting hot enough (although it has worked fine for 10 years), but all the other baths and showers on second and third floor of the house are working perfectly. What could be the problem? Thanks.

We've lived in our house for 10 years. All of a sudden, the shower in our master bathroom isn't getting hot enough. All the sinks, and other showers and baths on this levle