How can I fix my hot water after error code F22 Dry is flashing?

I have a British Gas 330+ condensing boiler, the error code F22 dry is flashing. It may be not enough water in the system as when I try to switch the hot water on manually the tank "echoes" and doesn't seem to be filling up, can anyone please tell me in lay terms & steps what I can do to rectify this? Thanks.