There are multiple reasons why a cooling and heating unit suddenly fails or gradually stops working correctly.
Anyone that pays attention is likely to note the changes as they set in! Know that there are reasons why the A/C suddenly stopped truly working.! Rather than panic, calmly stop and assess the potential causes.
First, start by checking the thermostat since this is one of the major issues, and you will be looking for dead batteries, which you can tell by replacing the existing ones with new ones. Occasionally all stopping the unit from truly working are dead batteries, but if changing thermostat batteries does not return everything to normal, the next site to check is the indoor switch. Find out if it was accidentally switched off, as this is likely to happen. Adjust it accordingly and check if normalcy returns. If not, proceed to check the outdoor unattach switch and adjust it. The air conditioner outdoor unit may be switched off for various reasons, some of which are not necessarily accidental. Reset this and check if everything returns to normal. If no changes happen, you must check for a tripped circuit breaker, a blown fuse, or a blown transformer! Most of these steps do not require the involvement of an HVAC business. You also want to inspect and disinfect dirty coils and replace dirty filters since they can cause the coils to freeze, which is not what you want. If doing all this does not work, then calling an heating plus A/C serviceman is sensible. There may be broken components that need the know-how of a professional cooling and heating plan expert.