Boil water notices are often issued because of disruption in power at the water treatment plant or due to distribution water pressure drop caused by burst pipes. In both cases, the water supplier cannot function properly in treating the water and must inform the public of unsafe water conditions.
If your municipal water supplier issued a "Boil Water Advisory" or your city was on a boil water notice, there are steps to take to ensure your household is safe throughout the duration of the advisory. You might also be interested in learning how a business can keep it's doors open during a boil water order.