Thawing partially accessible frozen pipes with a hairdryer
Your best bet is to prevent the pipe from freezing in the first place. Once the pipes are frozen, a hair dryer will heat up a copper pipe slightly and that heat will travel down the pipe melting the outside of the ice column and breaking it free and it will travel down the pipe. To melt it completely would take hours. A propane torch would do it much faster but they could be a fire hazard waving it around and then there's a chance of ruining a solder joint.
There are pipe heaters especially designed for preventing pipe from freezing. You wrap them around a pipe, plug them into a wall outlet and set the thermostat to the desired temp. They come in different lengths and operate on about 5 watts per foot.  See the picture below.

This one is from LingRan Industries, which I have no interest in.