Water pump runs constantly.

Hello all,
I am looking for guidance from all you experienced hands.

The water pump on our 1997 AS Harmony had been working fine but now whenever I turn the 12v on it runs constantly. No water comes from the taps, loo etc. I have tried knocking the tank in case it is an air lock but no joy. I have even tried sucking the water through! Still no joy.
Can anyone please suggest what to do next.
Thank you

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It helps us understand how to offer advice to you if you introduce yourself and describe your motorhoming/camping experience at the link above.

First question, is your fresh water tank full?

Thank you for the quick response.
I will fill it right up and give it a try again.

Ok, then check that any water heater drain plug/valve are closed and, if fitted, the vent heater valve is closed.

Oh. How embarrassing!

First rule for beginners:- never believe what the water tank gauge says.
I will fill out more details as time permits

Thank you again.