Hi
I have not had problems with my A/C yet, but I have dabbled with my Ford's a bit. I would doubt that the compressor itself has suffered a failure in that it worked a week ago. Perhaps the compressor clutch. With that said I would think the most likely to be refrigerant. (Note: suppositions follow
) There should be a pressure switch that signals the system to run. If the refrigerant charge is low, the switch will remain open and the system will not run. If you are a DIY'er I would purchase a simple Freon 134a guage set. It should have changeable ends to measure the low and high side. Take a low side measurement after the car has sat for a long while. The low side port should be on the aluminum tube on the firewall. Let us know what you find. As I recall in my '99 manual, for this initial test it asks "is the reading at least 30psi?" That would be a good place to start. If the pressure is good, then perhaps the low pressure sensor (clutch cycling switch) has failed, or again perhaps the clutch.
I am no expert and have not had an A/C compressor fail, but, with a full freon charge, and all other things normal I believe the system will engage and attempt to run even a failed compressor.