Sudden drops in water pressure may indicate plumbing problems that need immediate attention, or may be temporary disruptions at the source. Power outages, frozen pipes and problems with water mains can all disrupt water flow. Small leaks usually won't cause a pressure drop but those same faulty fittings could suddenly burst. Well pumps and pressure tanks may also be the source of the trouble.
During power outages, homeowners with wells will have no running water once pressure in the system is lost. Pressure should come back when power is restored. If not, check for a tripped circuit breaker or blown fuse in the pump's power supply. Wells with a low gallon-per-minute output can be temporarily pumped dry by heavy use. Most pumps should be turned off until the water level in the well rises again.
Drops in pressure might indicate a problem with the water main. Major leaks in supply side pipes may not be obvious above ground. Check the water meter when no water is being used in your house. If the meter is spinning, the problem is on your side and turning off the supply at the meter is advisable. The cost of repair and lost water will be yours. Fix any problems which are your responsibility before restoring service.
If you wake up in winter to a cold morning and no water, frozen pipes are the likely reason. Electric heat tapes wrapped around trouble spots may thaw pipes without damage, but expanding ice could fracture pipes or separate fittings. Check the problem spot after flow begins again and repair any leaks. If you experience an unexplained drop in pressure, always check for damaged pipes. Partial failures may not divert enough water to completely disrupt service.
Low water pressure after a storm could mean that your well pump was damaged by lightning. If fuses are good and circuit breakers are set, but the pump will not run, that part of the system may have to be replaced. A second possibility is the pressure switch, which automatically starts the pump when pressure drops to a preset minimum. Faulty switches may not reset at all and should be replaced by a competent electrician.
If pressure settings are incorrect, water pressure will regularly drop below usable levels and then return to normal. If an overpressure valve blows, water vents from the valve with the force of a broken pipe. If the tank's bladder -- the collapsible balloon inside the tank -- ruptures you will have water only when the pump is running. Pressure will fall almost instantly when the pump stops.
Individual fittings like shower heads and faucets may show reduced flow after months of use. Cleaning with lime and scale-removal products and replacement of internal filters should fix the problem. Improperly maintained or faulty water conditioners can also restrict water flow in the system.