I wonder if this was in an apartment building. We owned a condo in a 5 story (4+1) apartment building and because it was taller than the San Jose water system was built for, our building needed (electric) pumps to provide water pressure to the building (there were tanks on the roof). If we lost power, then we lost water.
Now that we have moved to a 2 floor detached home (also in San Jose) we do not have that issue, and everything is gravity fed.
Do you lose water in the whole building, or just those apartments above the water-line?
Usually these relatively low height kinds of top-tank systems lose water for the entire apartment building, because there's one pump to raise the water to the tank, which then passively provides the pressure (usually through pressure regulators at each floor if I remember right).
Larger buildings tend to have multiple independent systems
We happened to live on the top floor, so I don't have personal experience for the lower floors, but the communication on the (non official) group chat for the building always hinted that any water outages (we had a few non-power issues with the pumps as well) applied to to the whole building. But thinking back that could be an unfounded assumption.