If you switch the AC on and the compressor does not start, my first guess would be little or no refrigerant, how old is the car and when was the AC last filled?
Note it is advisable to leave the AC always on yes this will use slightly more fuel, but will keep the ac working for longer, I've just had our other car refilled it had only 30% left in( it leaks out over time, getting worse over time), and when was the cabin filter changed(every year)