Thaw it first
Set the thermostat to Off, or switch cooling off and the fan to On. Moving air over the coil melts the ice faster. A heavily iced coil can take several hours to thaw. Don't chip at the ice; the coil fins and copper lines are easy to damage.
As the ice melts, a lot of water drains at once. Check that the drain pan isn't overflowing and the drain line is running.
Find the airflow problem
- A clogged air filter is the most common cause. Replace it.
- Closed or blocked supply vents and blocked return grilles starve the coil of air.
- A blower fan that runs slowly or not at all can't move enough air across the coil.
- A dirty indoor coil restricts airflow and needs professional cleaning.
When it's refrigerant
If the filter is clean, the vents are open and the system still ices up, low refrigerant from a leak is a likely cause. Topping it off without fixing the leak only buys time. A licensed HVAC tech can find the leak, repair it and recharge the system.