Usually it's because the system is low on freon. But sometimes it can be an electrical problem. start with the basics are the fuses ok both inside the car and under the hood? The fastest thing to do would be to use a test light and see if you have power down at the compressor clutch. If you don't suspect low system charge. If you do then either the compressor and ac are working and you've got a different problem like with the blower motor. Or the clutch is trying to engage but slipping because it's worn out (or the compressor locked up). Hope this gives you someplace to start.