In order to thaw the pipes you can use a number of methods. The most common being a warm water bottle, a heat wrap (the kind found at pharmacies for muscle injuries) or just plain warm water.
The important thing is to make sure the water is warm, NOT boiling, as pouring boiling water straight on a frozen pipe could cause permanent damage. Equally, if using a warm water bottle or heat wraps, these should be applied over a period of time to allow the pipe to thaw slowly.
If you experience frozen pipes regularly, it is recommended that you get a plumbing specialist to have a look and possibly add more insulation to the exposed sections of pipe work as frozen pipes are one of the most common reasons for boiler breakdowns.