If the compressor is running, its capacitor and relays are fine. I suspect it simply isn't chilling anymore. The external heat exchanger should be quite hot to the touch while the internal one (accessed from inside through the ceiling) should be cold. If they aren't, it's dead.
If its out of freon, it has a leak. These are sealed units so a filler port would have to be added in addition to finding and fixing the leak. Most people find it wiser to just replace it. After all, its 20 years old.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL