There are numerous reasons why a cooling and heating unit suddenly fails or gradually stops working properly.
Anyone that pays attention is likely to note the changes as they set in.
Know that there are reasons why the AC suddenly stopped 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. You will be looking for dead batteries, which you can tell by replacing the existing ones with new ones. Sometimes all stopping the unit from working are dead batteries. If changing thermostat batteries does not return everything to normal, the next place to check is the indoor disconnect 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 disconnect switch and adjust it. The air conditioning 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 clean 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 HVAC technician is sensible. There may be broken components that need the expertise of a professional cooling and heating system expert.