Usually this is just a broken cable on the mast and can be repaired inexpensively and with not too much trouble. If the cable broke in the antenna, you need to remove the antenna assembly to remove the end of the cable from the drum so the new mast can fully retract.
Take a 17 mm wrench and remove the nut over the antenna where the mast comes out of the body. Turn the key on and the radio on. Pull out the old mast and as much of the cable as possible. Hopefully the chrome sleeve will come with the old mast. If you bend an antenna, this step is as far as you have to go for disassembly if you catch it before the cable breaks.
If the cable already broke, pull down the carpeting on the left side of the trunk/hatch to access the antenna motor. It comes out with the removal of the screws holding the bracket to the car. Fish it out of the hole and unplug the wiring and antenna lead. Be careful not to drop them down too far. Get the motor assembly on a bench and remove the bracket if necessary to access the 7 mm nut holding the round drum to the assembly, Remove the nut and carefully remove the drum. Watch out for the spacer shims on the stud the nut came off of or the drum may bind after reassembly. Pull out the white plastic drum and extract the remnants of the old cable. While you're here, clean the insides and relube with grease sparingly. Reassemble the drum and its cover and reattach the bracket if removed. Do not at this time try to install the new mast.
On NG 900s that the mast doesn't retract fully, but that shows no sign of damage, remove the clip at 2 and remove the drive wheel (1) Under the drive wheel ( and on top) there will be two washers. To remedy the retraction problem on those cars, remove one of the washers under the drive wheel.
Put the antenna motor back in the car and attach the wiring. leave the 17 mm nut off the top for now.
Extend the new mast and start the cable in the hole. You can do the rest alone, but an assistant makes it much easier. Turn the key on and the radio on, the motor should turn. Gently push and turn the cable down until you hear it scrape the gear in the motor. Wait for the motor to stop. You have to turn the cable around until the gear and the cable teeth line up ( are oriented the same way). Turn the radio off and the motor should pull in the new mast. Don't worry about the mast going all the way down this first time. Slide the sleeve down the mast and install the 17 mm nut and tighten it. Turn the radio back on, the mast should extend. Wait for the motor to stop, it usually gives three or four clicks after fully extended. Then turn the radio off and the mast should fully retract.
If you replace a bent mast before it breaks the cable, all you need to do is pull out the old mast and insert the new one as above.