When the well pump won't run due to a power outage or the municipal supply becomes contaminated or otherwise undrinkable, there are many sources of water available to you. Many of these sources need to be filtered, or otherwise treated before consumption.
There are many high quality filters available on the market today that will clean water beyond any imaginable contamination. Berkey & Katadyne are the major players , offering filters from personal size to larger capacity for family use.
Boiling your water for 5 minutes will also render it safe to drink. Boiling will reduce the taste and will not remove heavy metals nor other sediments. You can help this by filtering through a cloth before boiling and passing it back and forth between two containers afterward.
Commercial chemical treatments are also available, usually in tablet form with one tablet purifying 1 quart.
At home, you can also treat water with bleach at a ratio of 8 drops per gallon. Remember to use regular bleach with no scent or other additives.
As for finding a source of water, there are several options. If you are lucky enough to live near a stream or river, you're set. In most parts of Canada, winter will lay down a blanket of endless frozen water. Just melt & go!
You can also collect rain water from your roof by placing a clean container under the downspout of your gutters. Remember to pre-filter this source and be sure it is properly treated, a lot of nasty stuff can collect on your roof.
Solar stills are also an option, but output can be questionable and slow.
For a short term disruption, you can get water from your hot water heater. Remember to turn off the electricity or gas before draining the tank. This water should be drinkable straight from the spigot, but don't assume anything. Treat it before you drink it.
One last word, whatever method of treatment you use, be sure you can sustain it. If you boil, make sure you have plenty of fuel for your stove. If buying a filter, get the spare parts and store them with your other preps. For chemical treatment, get enough tablets or bleach to last a while. Remember the 2 + 2 rule. 2 quarts of water for drinking + 2 quarts of water for cooking and hygiene per person, per day. For hotter or dryer climates, increase by 2 quarts per day per person. Never use stagnant water that is not at least partly clear or obviously contaminated unles your life is in immediate danger, the risk just isn't worth it.