There are numerous reasons why a cooling and heating component 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 A/C suddenly stopped working. Rather than panic, calmly stop and assess the potential causes. First, beginning 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 component from working are dead batteries. If decreasing thermostat batteries does not return everything to normal, the next thing to check is the indoor remove 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 remove switch and adjust it, the air conditioning outdoor component may be switched off for many 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 heating and A/C business. You also want to inspect and disinfect dirty coils and update 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 and A/C serviceman is sensible. There may be broken components that need the expertise of a professional cooling plus heating method expert.