Wrap the pipe in or towels soaked in hot water.
If you have access to the plumbing below the tub, you may see visible frost or ice on the exterior of the P-trap. Safe Methods to Thaw a Frozen Bathtub Drain bathtub drain frozen
Remove the overflow plate (two screws). Shine a flashlight down. Ice visible means freeze in the vertical drop or trap. Wrap the pipe in or towels soaked in hot water
Salt lowers the freezing point of water, helping to melt ice faster. bathtub drain frozen
Before you start pouring solutions down the drain, confirm that ice is the culprit. Common signs include:
The "P-trap" holds standing water to block sewer gases. If this trap is located in an uninsulated crawlspace or near an exterior wall, that water can easily turn to ice.