There is ice on the air conditioner.

Switch the cooling off at the thermostat but leave the fan set to on. Moving room temperature air over the coil is what thaws it, and it takes several hours rather than minutes. Do not chip the ice off, and do not keep running it in cool mode, because a compressor pulling against a blocked coil is how a repair becomes a replacement.

While it thaws, change the filter.

Why an air conditioner freezes

The coil gets cold by evaporating refrigerant. It only stays above freezing because enough warm room air is passing over it. Anything that reduces that airflow, or reduces the amount of refrigerant, drops the coil below freezing, condensation turns to ice, the ice blocks the airflow further, and it runs away from there.

So the cause is always one of two families, and telling them apart is most of the diagnosis.

Airflow problems, which you can often fix:

A blocked filter. By far the most common.
Closed or blocked supply vents, or blocked returns behind furniture.
A dirty indoor coil.
A failing blower motor or a slipped belt on older equipment.
Ductwork that has come apart or been crushed.

Refrigerant problems, which you cannot:

A leak. Refrigerant is not consumed, so a system low on it has a leak somewhere, and topping it up without finding the leak is buying the same repair again.
A failing metering device.

The thaw, done properly

Cool off, fan on. At the thermostat. Not the whole system off, because the fan is what does the thawing.
Put towels down. A thawing coil produces a surprising amount of water and the drain pan may not take it all at once.
Wait. Two to four hours is normal, and a badly iced system can take longer. Checking every twenty minutes does not speed it up.
Do not use a hair dryer, a heat gun, or a scraper. The coil fins are thin aluminum and the tubing is soft copper. Both are easy to puncture, and a puncture turns a filter change into a refrigerant repair.
Change the filter while you wait.
Check the condensate drain is clear, since the melted water has to go somewhere and a blocked drain floods a ceiling.

Then restart and watch

Once every trace of ice is gone, set it back to cool and watch it for an hour.

Cooling normally, no ice: it was airflow, and the filter was probably it. Change the filter more often than you have been.
Ices up again within a day or two: airflow is still restricted, or it is refrigerant. If the filter is new and the vents are open, call someone.
Runs but blows warm and does not ice: likely refrigerant or the compressor.
Ice on the outdoor unit in cooling season: unusual, and usually refrigerant or a fan fault. Note that ice on an outdoor unit in winter on a heat pump is normal, and it defrosts itself on a cycle.

Why running it iced is expensive

Liquid refrigerant that has not evaporated can return to the compressor, which is designed to compress gas and not liquid. That is what damages compressors, and a compressor is one of the most expensive things on a system, often enough to make replacement the better option.

Which is the honest reason for the instruction at the top: switching it off for an afternoon is the difference between a filter and a new system. What the various repairs actually cost.

Keeping it from happening again

Change the filter on a schedule, not when you remember. Monthly during heavy use is not excessive with cheap filters.
Do not close too many vents. Closing off rooms feels efficient and starves the system.
Keep returns clear of furniture and curtains.
Get the coils cleaned as part of an annual service, indoor as well as outdoor.
Keep the outdoor unit clear of plants, leaves, and debris, with clearance all round.
Have an annual service. It is the cheapest thing here and it is what finds a slow refrigerant leak before it ices.

If the ceiling got wet

A frozen coil that thaws unattended, or a blocked condensate drain, can put a lot of water into a ceiling below an attic air handler. That is a water event with its own clock. What to do about water coming through a ceiling, and the 24 to 48 hour window that follows.

The short version

Cooling off, fan on, wait hours. Never chip or heat the ice. Change the filter while it thaws and clear the condensate drain. If it ices again with a clean filter and open vents, it is refrigerant, which means a leak and a technician rather than a top up.