There is a screen in the flush valve that gets clogged. Usually from the anode breaking down in the hot water heater. Disconnect the inlet hose and remove the valve. You may or may not need to remove the outlet hose depending on its length. Look in the inlet side and you will most likely see it caked with white crystals.Clean it out with a small pick. AND, it will happen again. AND, I know hot water doesn't feed the valve. BUT, trust me on this.
OOPS, HERK beat me to it.