AC blows hot air

It could be a number of reasons. It may be as simple as debris on the condenser not allowing air to flow sufficiently through the condenser fins. It could be a bad compressor. It could be insufficient refrigerant. It could be a blockage in the refrigerant system. You need to take it into a shop and have a system performance test run on it. While you could do this on your own with a manifold gauge set, it won't do you a whole lot of good if you don't know what the numbers on the manifold gauges are indicating.