Compressor failure or a leak in the refrigerant. With the engine running have someone turn on the ac. you should hear the compressor click on. Either way it needs to go to a mechanic as you need special tools to repair. They have dyes to inject to find leaks.
the air conditioner compressor may not be working, the refrigerant may need to be recharged, or the door inside of the HVAC unit that controls the blend of air may be stuck in the "heat" position. I recommend having a professional analyze the problem unless you have an intimate knowledge of car HVAC systems.
Your VW Beetle was made out of plastic parts by an undertrained work force in a thrid world country. Be glad your transmission has not yet fallen apart, like mine, and open the windows, if they still work.
The main cause may be that you are out of 134a refrigerant caused by a leak in the a/c system. It may also be caused by a faulty a/c compressor or a loose or missing drive belt. At any rate you should take it to a quailified a/c service center.
That's becasue your VW Beetle AC has run out Freon, freon is what make the AC blow out cold air, when your car is out freon causing the AC to blow out warm air, so your VW Beetle needs to recharge the freon for it to blow cold airs.