Never use an open flame (blowtorch, propane torch) to thaw a pipe. This creates a severe fire risk and can cause the water in the pipe to boil and explode. Never pour boiling water directly into a frozen drain, as the thermal shock can crack PVC pipes.
If you can see the frozen section (like under a sink or in a basement), use a gentle heat source to warm the pipe directly. best way to thaw a frozen drain pipe