It could be several reasons. First thing to do is check all the fuses. Lokk under the hood and under the dash. If they are all good, it could have a leak and lost all the refrigerant. You can check for power at the compressor if you have a test light. Check the wires going into the clutch with the key on, engine off and the ac turned on. If you have power then you've probably got a leak somewhere.
If it does blow just not cold air you should see if it needs to be recharged
check your thermostat first
Freon is too low, or too high.
An air conditioner blows out heat
Portable air conditioner will not blow cold air. What to do?
Replace your cabin air filter.
Inspect all the vacuum lines, you may have one cracked and you are loosing vacuum to operate the HVAC system.
how can i solve the problem
you would need to add freon
gas leak or wrong wiring
diagram for 1999 chev subarban air conditioner
Coming out of the vents around 43 degrees