First, you can ask the reason to solve it. Distant relatives are not as good as close neighbors, and it is not cost-effective to hurt the gas. If the communication fails, you can find the local neighborhood Committee or property to find out why the tap water valve is turned off privately.
You can negotiate with the house downstairs and ask him to stop this misconduct. If the other party doesn't listen, you can bring a lawsuit to the court and ask the court to order the other party to stop the infringement and compensate your losses.