Thanks for the advice, Joe and Tom. The freon was checked a month ago when the downstairs unit blew a capacitor. While the repair guy was here, he checked the level on both units (simple to do, might have saved me some trouble, and might have generated more business for him- all round good idea).
The fan motor is getting power, but it is either siezed or burned out. I'll probably remove it first to see if there's anything obvious that caused it to fail before I replace it (and let the same thing happen again). Thanks again for the advice!