Shut the electricity off at the breaker or turn the gas valve to "pilot" depending on the type of water heater you have. If you know you have a clog, then it is assumed the water intake is shut off and the tank was in the process of being emptied.
Try closing the temperature relief valve and opening the cold water intake while the hose is still hooked up to the drain valve. This will cause a rush of clean water that may push the clog out. If it doesn't work, shut the cold water intake off again. Open the pressure relief valve and wait until the water is not hot.
When the water has cooled off, carefully take the garden hose off of the valve and try placing a hard piece of wire that will bend in the valve to break up the clog. Have a bucket under the drain and be ready to shut the valve off once the clog has cleared. Then hook the garden hose back up and finish draining the tank.
If that doesn't work, you will have to remove the drain valve. If this is the case you will need buckets and/or trash cans to catch the water that is left in the tank. If it is high off the ground you can use two trash cans. Put one under to catch while you empty the other. To remove the valve, unscrew it with a wrench. Then use a long screwdriver to put in the hole to break up the clog.