Do you mean that the large tank in the loft is actually empty? If so, the hot water will fail soon as well as the cold - but often the cold is supplied direct from the water mains. Anyway, no matter. In the loft tank you'll see a float valve with a plastic ball on the end. Work this gently up and down a few times and see if the flow restarts. If it does, it will be furred up and its best to replace it. If nothing happens, then it definitely needs replacing!
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