There are many reasons why a cooling and heating device suddenly fails or gradually stops absolutely working consistently, anyone that pays attention is likely to note the fluctuations as they set in.
Know that there are reasons why the A/C suddenly stopped absolutely working.
Rather than panic, calmly stop and assess the potential causes. First, start by checking the control device since this is one of the major concerns, you will be looking for dead batteries, which you can tell by replacing the existing ones with new ones! Sometimes all stopping the device from absolutely working are dead batteries, however if changing control device 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, however the a/c outdoor device 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 fluctuations happen, you must check for a tripped circuit breaker, a blown fuse, or a blown transformer, and most of these steps do not require the involvement of an HVAC business. You also want to inspect and clean 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 HVAC supplier is sensible. There may be broken components that need the expertise of a professional cooling and heating system expert.