There are many different reasons why your compressor is not turning on when you turn it on. The most common reason is that the battery is dead or there is no power available to turn on the unit. There are several things that could be causing this problem such as a short or dead circuit. If you cannot turn on the compressor then chances are the battery is bad or the compressor is not working.
Some of the first things that will show up on the gauge while you are trying to turn on the unit are the blisters and ripples that appear on the filter. As soon as you see one of these you can begin to try to turn the engine and see if this does anything. If the unit is not turning on at all then you need to check the battery for any problems. If the compressor is not turning on when you turn it on then you might have a dead battery. Dead batteries are very common causes of not being able to turn on the compressor.
You may have to physically remove the battery to check it. If the battery is out of warranty, you may be able to replace it with a new one. If the battery is beyond the warranty then you may have to replace the entire unit. You may have to look in the house to find the battery and remove it. You do not want to cut into the compressor itself because that would damage it and you would have to buy a new one. It is better to fix the problem before you replace the compressor.