Most people have a pumped water supply. Pumping water around the country is such a high user of electricity that should the grid fail, you will not get water out of your tap. (The sewage systems often rely on pumps, too.)

In fact, in California at least 6.5% of the state electricity is used for pumping water, over 15,000GWh each year. That's roughly equivalent to the total yearly output of two 1000GW nuclear reactors.

For a particularly sobering view of what life will look like when the grid fails, see this National Academies congressional testimony. The relevant part is reproduced below:

While the report does not speculate on the extended consequences of such an event, I have been asked to do so here and so offer this as personal opinion. Because our critical infrastructure is so completely integrated, with the power out for even a day or two, both food and water supply soon fail. Transportation systems would be at a standstill. Wastewater could not be pumped away and so would become a health problem. In time natural gas pressure would decline and some would loose gas altogether. Nights would be very dark and communications would be spotty or non-existent. Storage batteries would have been long gone from the stores if any stores were open. Work, jobs, employment, business and production would be stopped. Our economy would take a major hit. All in all our cities would not be very nice places to be. Some local power grids would get back up and so there would be islands of light in the darkness. Haves and have-nots would get involved. It would not be a very safe place to be either. Marshal law would likely follow along with emergency food and water supply relief. We would rally and find ways to get by while the system is being repaired. In time, the power will start to come back. Tentatively at first, with rolling blackouts and then with all it glory. Several weeks to months have passed, and the clean up would begin. This is one man’s opinion.

The testimony above describes what would happen with an infrastructure breakdown caused by a determined team of terrorists, thus it assumes the grid will come back up once the damage is repaired.

But what do we do when the grid is too poorly maintained to come back up?  

Capturing, storing and filtering water will become very important.

If you have access to a rooftop, you can redirect rainwater to some sort of storage vessel. If you can't bury the vessel, it will have to stay above ground. With some water barrels, you'll require a Siphon Pump and possibly a 10' or 25' fresh water hose.

You may want to consider installing a greywater system that allows you to reuse water from showers and sinks for other purposes. Don't confuse that with reclaimed water, which comes from sewage systems.

Make sure you have a water filter and plenty of replacement filters in case your tap water becomes unreliable.