There is a stiff plastic cable that extends from the bottom of the mast. It engages the motor and winds around in the motor casing to make the mast go up and down. The symptom you describe usually means that the cable has broken.
You can try one thing. When the antenna is stopped 6 inches from home turn on the radio and then immediately turn it off. The antenna should go up a little bit then back down. If it has just lost track of where "down" is, that should cause it to go the rest of the way down if its not broken.
If that doesn't work, then you probably need a new mast. Remove the cap around the antenna on the top of the fender and turn on the radio. The mast will go up and keep on going all the way out. Turn off the radio after you get the mast out. Look at the cable at the end and see if it is has broken off. If it has, turn the radio on and see if the broken piece moves up in the shaft. If it does, you should be able to fish it out with something. This is the ideal scenario. Just get a new mast from the dealer (about $40) and feed it back in. Turn the radio on, then off and it should pull it back down & you're done.
If the broken part has retract0ed all the way into the motor housing, (unfortunately, the likely case) you will need to take the antenna motor out, open the casing and remove the broken section of cable. Put it back in the fender, then feed in a new mast as above.