The first thing that you need to do is to find the frozen section of pipe. You can do this by checking the flow of water from all appliances, this should help you to identify roughly where the problem is.
Once you believe that you have identified the section of the pipe that is frozen, it is time to play detective. Look for clues such as; Draughts from air vents next to pipework or an unlagged section of pipe or pipework next to a vent in the roof space – these are all possible causes.
Inspect the pipe before attempting to thaw the blockage and also any nearby fittings. It is possible for the pipe to have split, or the fittings to have been pushed apart by the expansion of the ice.
If you were to melt the ice at this point, you will just end up with a leaking joint! If you find such damage, the water supply to this section should be turned off first, and preparation made to contain the leaking water once the pipe is thawed.
Apply heat gently to the affected area using a hair dryer or hot water bottle. A hair dryer, however, may only be used if you are certain that the pipe / joints are not damaged or split. Never use an electrical appliance in the vicinity of water. If you use a blowtorch, guard against fire and apply only gentle heat. Excessive heating will cause soldered joints to fail.
Open the tap on the affected pipe to allow water to flow as soon as the ice begins to thaw. If you are unsure as to the location of the ice blockage on the length of pipe, start nearest to the tap and gradually work away from it.