We just moved into a house and have NO hot water in the master bathroom shower, however, we do have hot water in the sink. What could be the problem?

Sounds like some trash is blocking the inlet ports of the ncartridge. What's the brand name of the shower valve? One handle or two knobs? If a one handled valve turn the water off to the valve and open it up. Remove the cartridge and check and clean the inlet ports at the bottom. While you have the valve apart turn the hot water back on for a few seconds to flush out the supply. You should have a strong stream. If not click on back and tell me about it.. Good luck, Tom