What to check, and what to shut off.

Before you call, run these.

• Thermostat set to Heat, and the target above current room temperature
• Furnace switch near the unit is on, and the breaker has not tripped
• Filter is not clogged. Replace it if you have a spare
• Oil furnace: check the tank gauge is not sitting on empty
• If the burner locks out repeatedly, note the error light pattern before you reset it

That error light pattern saves real diagnosis time. Write it down or photograph it.

Nothing coming out of the tap.

• Open the nearest faucet slightly to give the thaw somewhere to go
• Shut the water off at the main if you see a bulging pipe or an active split
• Never use an open flame to thaw a line
• Photograph any split before you shut everything down

Thawing an accessible line starts around $169, plus repair if the pipe split.

Ice on the outdoor unit.

• Light frost in cold humid weather can be normal
• A defrost whoosh once or twice an hour is normal
• Shut it down if the coil is a solid block of ice
• Shut it down if indoor airflow has gone weak, or you hear grinding
• Clear leaves and drifted snow from around the unit if you can reach it safely

Open a ticket with a photo if you can. Blocked airflow, a failed defrost board, and drainage all look similar in words and different in a picture.