This needs more study.
There are at least three problems that would cause this. Meanwhile don't force the antenna down so you don't snap the plastic tail inside it. Extend it back up gently by hand til you get more info.
3 possible issues. Address them in order.1. Antenna motor-drive not powered - No power or no switching signal from the radio to operate the antenna. Blown inline fuse, bad ground, prev owner installed a radio that doesn't switch the antenna, botched wiring, etc.
2. Antenna motor-drive broken - good power, ground and switching does not make antenna motor move
3. Antenna motor-drive works - antenna mast's internal geared-plastic tail snapped in two
Address these in order.
Starting with #1 - Did your antenna drive ever make a sound since you got the car? You should hear it running for a few seconds - or trying to run - when you turn the radio on-off-on with key on (engine off), even if the mast or the drive are busted.