most systems have a pressure switch that stops the cmpressor coming in if the is no gas it the system. The gas circulates the oil around the system that keeps everything working properly hence no gas the compressor in inhibited so as not to cause any damage.
If you pressed the valve in and there was no gas coming out the systen is empty might need a regas.
The worst thing for air con is not being used the seals dry out and the gas leaks past them. so even in winter get the ac cranked up now and again to keep the system working, plus the ac is the best way to clear a misted up windscreen