Has your water stopped getting hot in your shower or bathtub faucet? Odds are it's because your Posi-Temp Cartridge isn't working anymore. This can happen over time, especially when the faucet is not often used, which can allow for mineral build up.
You have the option to have a plumber come out and fix this, which may run you from $300 to $500. Another option is to buy a nice, new cartridge and a good cartridge removal tool and any hardware store for about $50. YouTube is amazing and you can learn to change the cartridge yourself in less than an hour. It can take a little longer if the old cartridge is particularly hard to remove, but that's why you buy a remover to help you do it.