Most likely the mast is just stuck but you'll need to test it in the car or on the bench to be sure. I would get down under there while someone turns on the radio and listen for any noises at all. If you hear anything, most likely the mast is stuck and binding up the motor.
I fixed my '56 Cadillac's antenna by carefully extending the mast manually on the workbench and cleaning and lightly lubricating the cable that's inside the mast. After carefully cleaning it and rewinding it, it goes up and down smoothly.
If you don't want to mess with it or if the motor is shot, etc, there are a lot of universal electric antennas that are available that you could probably install without too much difficulty.
Here's a How-to guide that more or less mirrors my experience with the Cadillac.
https://www.familyhandyman.com/automoti ... /view-all/Or you can buy one of these and replace the whole thing.
https://www.ebay.com/sch/i.html?_from=R ... a&_sacat=0There used to be a guy that rebuilt power antennas on a list of Cadillac parts vendors that I created but he's no longer in business.