Welcome to JustAnswer.
Sorry to hear about your problem.
If this is in the shower/ tub only, you will need to take the control valve apart, to flush out any particles that are most likely clogging the small holes in the valve. Below is an example of a control valve and it's parts.
Your valve may have seats and springs or a cartridge. Your hot water pipes may have been disturbed or you have mineral build up in the water heater but the hot water seat is most likely blocked in the valve. If you have a cartridge, it may need to be replaced.To access the valve, you will need to remove the handle and the face plate. You will then see screwdriver stops where the water enters the valve. If not turn off the water. Now unscrew the bonnet nut and pull out the stem unit assembly. This will expose the seats and springs. Remove them and check the opening for crud or blockage. Now, while the valve is open turn on the water and flush out the supply line. You should have a full stream. If so, reassemble. If not then the blockage is further downstream and you may have to call in a plumber to further diagnose the problem.
In addition, remove the shower head and clean it thoroughly and then reinstall.
If you need further help or clarification on this answer, please do not hesitate to reply to this post.