I doubt the mechanic would've disabled it elsewhere. Could be that the refrigerant charge has leaked down enough over time that the low-pressure switch won't let the compressor kick on. I don't know why that would stop the light in the button from coming on, though. Could be that's on the circuit controlled by the pressure switch, or you could be correct about an electrical fault elsewhere that just isn't telling the aircon to engage in the first place. I wouldn't expect a faulty relay to explain the light not coming on, but I don't know how Nissan wired it up. The service manual should have the wiring diagram (probably in HA, if not try EL), and some troubleshooting ideas as well.
My first check would be shorting out the low pressure switch. If the compressor starts with that bypassed, that's a pretty good sign that the gas is low and needs to be refilled (and the leak fixed, if you can find it). If you've got no voltage across the low pressure switch, you've got an electrical fault elsewhere.