Hot water not working in shower

Hi all,
I hope someone can help me diagnose the problem with our shower:

Our house is only 3 years old, we have a third washroom that we rarely use. This shower has not been used at all over the last year or so. A few days ago I tested it, and noticed that the hot water is not working in the shower.
Its a Moen One Handle Shower Faucet. When I turn the knob the cold water runs fine, but as I get past the 1/2 way mark (to the Hot) side, the water pressure decreases. When I have the knob turned all the way (full hot) there's nothing coming out from the faucet (or shower).

What could cause this?

Is something jammed/stuck because it hasn't been used in a year?

Any advice/suggestions would be appreciated?

Newer valves are required to be antiscald. There is a sliding valve in there that moves based on pressure differences. It is likely that this is jammed in one position, preventing the hot from running.

If you check out Moen's website, you can probably see it (often called a spool valve). My guess is that if you get a little plumber's grease, you can reinstall it and it should work. Check out their website to see if they have troubleshooting tips. One of the plumbers that works with these things will have an idea.