Water that barely moves or backs up in sinks and tubs during winter.
If the pipe is accessible but hard to heat directly (e.g., a pipe in a tight crawlspace): how to unthaw frozen drain pipes
If you get even a trickle of water, you can then help melt remaining ice: Water that barely moves or backs up in
This is the most effective method specifically for sink/tub drains, but it is messy. but it is messy.