There could be a lot of reasons. Hot water entry point clogged, hot water heater out, Hot water line broke or not hooked up. If this problem has only happened since the shower has been replaced or serviced, the chances are that there is a hot water safety that just needs to be adjusted. Take off the handle, look at the assembly, you should see a plastic sleeve or knob that can be turned. This knob stops that handle from turning past a certain point which restricts the flow of hot water.
Could be a few things... Most likely is that you have an anti-scald safety adjustment under your shower handle. Faucets that come with these things usually don't have any visible indication on the finish trim, so you can check the manual (if you have one) or just remove the handle and see what's there. There are a number of different types, but most have some sort of plastic ring with a bunch of teeth on them that fit into a ring ( also with a bunch of teeth). You remove the ring and rotate a bit to give hotter or colder water. Another possibility is that somewhere else in the building, there is a cross connection between the hot and cold water. This allows the cold water to flow into the hot water pipe when the faucet is opened, giving mixed water that is tepid at best. You have to find this by trial and error usually. You can try shutting off water to various places ( washing machines, outside or inside faucets, equipment with water connections, etc.) and find out which unit is causing the problems. One other option is the faucets themselves. A lot of shower faucets have an internal pressure balancing device. They prevent the water form turning suddenly hot or cold when the water is used elsewhere in the building. Great idea, really, but like any mechanical device, it can malfunction. This is a bit more than most homeowners are willing to tackle- they usually call in a plumber.
There should be two valves on or near the top of your water heater. One is for the inlet (cold water) into the heater the other is for the outlet (hot water) out of the heater to your house. They must both be open ( turned to the left or counter clockwise).
The faucet opening screw thread is stripped, the faucet washer has collapsed, the pipe is furred up, the boiler has a problem
Probably large air bubbles accumulated in the supply pipe. Bleed them out.
Is it switched on? is the filter clean? is the element and thermostat working properly?
If everything else has plenty of hot water, shower valve may need adjusted or replaced.
You repair or replace the shower control unit.
I'm no plumber, but I would imagine that (if the hot faucet is giving you cold water) that the hot and cold pipes leading into the bathroom have been somehow switched.
Try cleaning out or changing the aerator on the bad one.
cold water tub facuet is corroded bye hard water builtup and needs to be replaced. Hot water is okay. Its the right one with the cold water coming out
The shut off valve under the sink, the faucet itself, or most likely, the screen in the end of the spout is plugged. If neither hot or cold is coming out, it is kind of unlikely both shut off valves and both sides of the faucet would go bad at the same time. The screen being clogged would stop any water from coming out. The screen just unscrews out of the spout.
Obviously hooked up wrong, or the faucets are mismarked . Usually cold on the right, hot on the left.
Assuming that you tuned the faucet to hot and let it run until the hot water should have reached it, you may want to check your water heater and make sure that it is working.
Odds are that the cold water lines to the faucet you are trying to use are shut off but the hot water lines coming from the water heater are not. That is why you have hot water but no cold water.
If there is a shut off valve under the sink, shut it off and take the line off going to the faucet. Turn the valve on slightly to see if there is water going through it. If there is, the problem has to be in the faucet. Either the washer in the hot water side has come off and is plugging the faucet, or if it is a single handle faucet, the cup is bad. Either one is easy enough to fix. Message me back if you need help fixing it.
Employ a plumber.
That's just how long it takes to get the water that is sitting in the pipe out. Unless you have a circulating pump on the line so that there is always hot water at the faucet, it always takes a bit of time for the water to get hot. The hot water is in the tank, not the pipe connecting it to the faucet. There is nothing in the pipe to keep the water hot.
Usually from a hot water tank.
Do you have hot water at other fixtures in the house? If not you have a water heater problem. If yes the Shower valve needs to be rebuilt if it is a bressure balanced faucet the concentration should be on the pressure balancing spool area of the faucet.