Heat is a type of energy. Cold is the absence of heat. Air conditioners are “heat pumps” - they pump heat energy from inside the house to outside the house, which results in the house getting colder.

Heat is pumped using refrigerant, which is a special fluid which absorbs heat when gaseous and under normal pressure, and releases it when put under pressure which turns it into a liquid.

There are two parts of the air conditioner - the outside part and the inside part. The outside part contains the compressor (which compresses the refrigerant) and the condenser (which forces the refrigerant's heat into the air with a big fan) The inside part has the evaporator (which releases the pressure to allow the refrigerant to pick up more heat using another fan, blowing out cold air as the key side effect)

The outdoor unit is outside the house. The indoor unit is in the attic above the back of the hallway. The vent in the hallway ceiling is part of the evaporator which pulls the air to be cooled into the evaporator. The cooled air is then distributed to all the vents. The two units are connected by two small pipes which carry the refrigerant. There is no actual air exchanged between the two units.

The inside and outside units are on different breakers, so it's possible for one side to stop working.

The air conditioner only cools air when the compressor is turned on and the refrigerant moves through the system. The thermostat controls this. You set the thermostat to a desired temperature. The thermostat knows the current temperature. If current temperature is above the set temperature, it turns the compressor on. If current temperature is at or below set temperature, it turns the compressor off.

The current temperature is measured at the thermostat unit, in the hallway. This is affected by things like whether the bedroom doors are open (which can let out cool air and drop the temperature in the hallway) and whether the sliding doors are open (which lets in hot air which raises the temperature in the hallway).

Only set the thermostat to the desired final temperature. Setting it below the desired temperature will not cool the house more quickly, it will just make it colder when it finally does cool down.

The evaporator (indoor) fan can be set to “On” or “Auto”. When “On”, the fan always stays on, which circulates air throughout the house. This can keep the air from getting stuffy and temperatures in some rooms stagnating. When “Auto”, it turns on/off with the compressor. This uses less electricity and is silent when the compressor is off (which may be desireable)

Make sure the thermostat set temperature is set well below the measured temperature.

Go outside and see if the compressor unit is running. If it's not, check the breaker box in the garage to see if the A/C breaker tripped. Reset it and see if the compressor runs.

Feel the air flow all over the house. Is it cooler in some places than others?

Check to see if the filter in the vent in the hallway is too dirty. Remove the filter and let the a/c run for 10-15 minutes without it. If the air starts cooling, replace the filter. There are extras in the storage room in the garage.

Air conditioners can only drop air temperature so much. The hot air to be cooled is pulled in from the vent at the back of the hallway. If this air is too hot (due to outside doors being open or bedroom doors being closed and not letting cold air back into the hallway), the effectiveness of the cooling will be impacted.

Is the fan setting on the thermostat set to “On”?

Check the breaker box in the garage to see if any breakers have tripped.