Fill a hot water bottle (not boiling) and press it directly over the drain opening. Alternatively, pour ½ cup of rock salt or table salt into the drain—salt lowers the freezing point of water and can slowly melt the ice.
If the blockage is accessible via an access panel: frozen bath drain
To effectively resolve a frozen drain, one must understand the mechanics of how water freezes within a plumbing system. Fill a hot water bottle (not boiling) and
Can a tub drain pipe freeze and how to prevent or fix it? - Facebook frozen bath drain