no hot water in a single handed shower with low pressure
Since you do have hot water coming out of other faucets it is most likely your shower valve.

Many of the single handle shower faucets have a cartridge valve that is very easy to replace. The cartridges are specific to the faucet so you will need to look up your model. I had a similar problem with mine. The cartridge cost around $60 and took about 10 minutes to replace.