Pour a pot of hot (not boiling) water down the drain. If it sits there for 10 minutes then suddenly drains with a "glug," it was almost certainly ice melting. How to Thaw a Frozen Tub Drain (Safely) Warning: Never use an open flame (propane torch, lighter, heat gun on high) on metal or PVC pipes. You can start a house fire or melt plastic fittings.
Few things are more frustrating than stepping into the shower, turning on the water, and watching it rise around your ankles instead of disappearing down the drain. If you live in a cold climate and the temperature has dropped below freezing, you may be facing a common winter plumbing problem: a frozen tub drain. tub drain frozen
Stay warm, and happy bathing.