I have hot water in sink but not in the shower

Tub and shower diverters, have tempering adjustments to allow the showers to be lower in temperature, to prevent scaulding. It is quite possible that your tempering adjustment on your diverter is allowing too much cold water, and not enough hot water.

Or more likely the mixing valve for the shower has clogged with some debris in the hot inlet. Did this just slowly start one day recently? Shut off you supply, open the valve body and clean. If you attach a picture or give us a brand name we can be more specific with cleaning instructions.