No hot water from shower after no use for 3 months. Sink and other bathroom are fine. It is a Kohler single handle fixture that is 2 years old. Spent the last 3 months replacing old drywall with concrete board and replacing toilet and floor and setting new tile. I never touched the fixture other than to remove cover.

Sounds like debris caught in the inlet port of the mixing valve. Remove the cartridge and clear it. With the cartridge still removed open the supply line for a moment to clear any additional debris, a big towel held over the valve body opening should keep you dry. Grease the valve with plumbers grease or silcone jel and reinstall, better now?

Thanks very much. I will give it a shot.