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...
Does your compressor clutch run?(you should hear it click in when Ac is turned on) If so you may have a vaccum line unpluged or broken located under the dash arround the hump at the fire wall. Or,...
Most commonly, the compressor is not working, the evaporator is blocked by dirt, the condenser motor isn't woring. A trained technician needs to evaluate