Shower water used to be hot, now it's cold

My downstairs shower worked fine until recently, when I turned the handle to the left for hot water, it stayed cold. I ran it for the longest time and still no hot water. Could it be a calcium build-up? If so, what can be done to correct it? The shower faucet unit is a "Touch Control" by Moen. Thanks in advance for your help.

How old is it?

After I checked the hot water from the heater is fine and that the shutoff to the shower is not off, I'd disassemble the shower trim and inspect the mixing valve & cartridge.

If there's nothing obvious in the valve - like corroded o rings or build up or blockage, then I wonder if yr cartridge might need to be changed.

Depending on how old it is, it may be a pressure-balanced valve. This has a sliding spool valve in it that adjusts the flow when the pressure changes so you don't get scalded when say someone flushes the toilet. Especially if you had the water off to do some work, that spool valve may have stuck in the position where it is now blocking the hot water. Depending on the brand and model, you may be able to remove it, clean it up, lubricate things, and return it to like new condition. It may require replacement. If you can figure out the brand and model, you may be able to look up the manufacturer's maintenance instructions on the web. Depending on the valve, you may be able to get free replacement parts.