What are the steps involved in flushing a hot water boiler?

I flushed our hot water boiler on Wednesday by opening the drain valve until the water ran clear and then closing it again. Now it seems there's no water pressure in the boiler, and our radiators are cold despite the thermostat indicating that the heat is on (I've since shut it off to save gas until I figure this out). Is there another valve I have to open/close to refill the tank?