Check pressure in the system first. If it's low you do have a leak. Check with engine off. Pressure is dependant on ambient temperature. About 70 psi on a lukewarm day. A/C won't run if it's below 20 - 30 psi. If you can't find the leak outside (at night) look inside at the evaporator.
If there is pressure and the clutch is off, check the fuse and the low pressure cuttoff switch. If the clutch is on you should also check the temperature mixing door in the ductwork to make sure it moves. This door is moved by an electric motor an sometimes fails.
