6 Hours Ago
Cold tank must have drained for that to happen surely? Normally happens sometimes when replacing the cwt, - water stays in the cold feed level with the hot cylinder, then first air & then water above that. Just putting a cold supply (gravity or mains) through to a hot supply for a few seconds will cure it & you hear the bubbles going up into the cwt. Holding your hand over the outlet of the older mixers on a bath say & turning both taps on will sort it.
Check incase the ballvalve is blocked on cold tank.
If the cold feed to hot tank is 28mm, it will be much less likely to airlock, especially if it has a short run or is not horizontal for a fair bit in attic. 3/4" pipes were prone to airlock easier.