water heater problem!!

OK..so I turned off the water heater's cold water valve, and then turned it back on, now there is a loud water hammering sound every time I try to turn it back on. So basicly the cold water isnt going on into the tank in the water heater, therefore no water to heat up..therefore no hot water...so i need help.
once again.. I turn the cold water valve from off to on and I get a loud water hammering sound all through the pipes. I need some advice or help..thanks alot.