Th hot water side of single handle shower faucet does not work, cold side works

remove and turn cartridge 180 degrees then re-install

change the cartridge

Hot on left.
Cold on right.
Some single handle faucets have bleed-over where both hot and cold enter mixing valve each time handle is turned on, even when faucet calls for just cold or just hot.
Otherwise maybe a do-it-yourself plumber got pipes backwards... or maybe the tub used to be on other side of wall, and tub was moved during remodeling but pipes were not changed.

To fix problem:
First of all, turn off shower faucet. Turn on Hot at sink and put ear against shower faucet listening for sound of water flowing through the mixing valve.
If mixing valve is working fine, you need to open wall and investigate more.
For example if you have access door, reach in and feel which pipe is hot.
Pipes might need to be reversed if problem comes up in home inspection, otherwise the problem will not harm anything.

If I understand correctly, you have single handled shower faucet.
Normally there is a wait for hot water to arrive at faucet.
But now hot water is already at shower faucet.

Single handled faucets have a mixing area inside valve where hot and cold come together before going into sink.
This mixing valve can have crossover where the hot and cold can pass through to each other even when faucet handle is off. When single handle faucets get old, this can happen.
How it happens:
For example another faucet is turned on inside house, and pressure differential between hot and cold causes hot water at shower handle to pass through mixing area and into cold water line.
So if another faucet has been run, then that lets hot flow to faulty single handled shower faucet.
To confirm this is the problem: turn on other faucet and put ear against shower faucet to listen for moving water.
This condition may also signal there is a water leak.
The problem is not harmful, but will draw cold water into water heater each time, and is energy wasteful.
Replace faucet or have plumber repair problem.

There are answers to this question already posted here. work on your search words on Google

if u have the paperwork for the unit--in it will explain how to adjust the anti scold settings single handle units are shipped in cold setting u have to adjust the hot -- remove handle and on shaft will see blue and red clips adjust the red till hot water is to ur liking replace handle

Can you remove handle , behind handle there should be a dial to move to increase hot or cold supply.

Remember where you start and remove it and replace a 1/8 to 1/4 turn counter clockwise, just put handle on try it and see if it changes and adjust accordingly.

adjust ur anti scold the setting is set to low when single handle shower units are shipped they are set in cold mode for safety ur install/instructional paperwork explains such and also advises on how to adjust---remove handle and on shaft will see a blue and red plastic clips that are adjustable slide towards u and move red for more hot water test water continue till comfort level is reached for hot water

You can try to rotate the first plastic ring to the left of center a few teeth, this will let the handle advance alittle more to the hot side.

change the cartridge it is bad or defective.