AIR - If you're on a public water supply, they may have been working on the lines nearby. If so, just run the facuets until the air is gone.
If you're on a private well and pump, there may be a leak in a line on the suction (well) side of the pump that is allowing air into the line, or there may be a leak in the prssure tank bladder (which means replacement).
HOT WATER - If you have a standard electric water heater, you probably have a bad lower element. Test it by turning the power OFF, disconnecting the wires to the element, and then test the element screws with an ohm meter...no conductivity...replace element.