Hot Water Heater Drain Faucet
Hopefully, while you had the tank drained, you also flushed it. This is done by draining and then turning on the supply valve with the drain valve still open. The diptube, which takes the cold water to the bottom of the tank, will then clean the sediment from the bottom of the tank. Since the sediment is heavier than the water, it will sit on the bottom while draining. You need to get it "flowing" so it will drain.
If you have never made a mistake, you haven't done much.