Sounds like the emergency shut off valve or the line to the hot water supply is clogged or the valve is in the off position. If clogged, sometimes just working the valve vigorously for a few minutes will free it up. Othertimes replacing is the valve is only solution. If the line is clogged, try opening the line at both ends and apply a little air pressure to clear the line.
Delta Single Handle Shower Has Good Pressure But No Hot Water. Every Other Faucet In House Works Well. How Do I Fix The Hot Water In Shower?
There is an adjustment you need to take the handle off and you will see a ring that has marks on it you need to adjust that for more hot water to flow.
It's probably plugged by rust. Get a parts kit, open up the valve and flush it out, paying attention to the hot water side. Search the web, there are pictorial step-by-step accounts.