The antenna mast (the part you see from the outside of the car) is a telescoping metal post with a plastic cable inside. This plastic cable is longer than the antenna when fully extended and has small teeth on one side. The cable feeds down into a spool attached to the antenna motor (imagine a powered fishing reel). In an antenna that is stuck in the up position, the cable has snapped just before the antenna was fully extended. The big piece of the cable (part inside the shaft) is lodged (imagine the barbs on an arrowhead) against the openning of the metal tube the shaft sits in when retracted. There is no way to force it back down. The small piece of the cable (still attached to the motor) gets ground around by the gears making a lound noise. It can also come free of the teeth, so the only sound is the normal motor sound.
My antenna recently broke in the extended position. I ended up replacing the entire motor and mast assembly, because I was able to buy it cheap from a fellow TI member.
New masts by themselves cost $60+ from Acura or $20 - $30 on ebay. You can change the mast from the outside of the car by removeing the silver nut at the base of the antenna. I used a flat-head screw driver and a hammer. Just tap lightly in a counter-clockwise direction and it will come off. After it's off, you can remove the old shaft and feed in a new one (takes two people--1 to feed and 1 to turn on the radio). You can also remove the old shaft, compress all the way down, and cut out the plastic cable. This will solve the antenna up problem, but will leave you with and antenna that is always in the down position. I would have done this as a temporary fix, if I hadn't found the cheap motor/shaft so quickly.