Remove the drain valve, or drain fitting if it is one that is connected to a straight pipe from the tank, and replace it, temporarily, with a gate valve or ball valve. Then use a screwdriver to dislodge the material in the bottom of the tank, next connect a hose to the that valve and open it while the water pressure is turned on to the tank. once flow starts turn off the water to the tank and lift the lever on the safety valve to let air into the tank. You may have to repeat the process a couple of times before the water will drain properly.