Jim,
You didn't provide enough info for anyone to be helpful. How old is the trailer? Has the hardware ever been lubed? Have you tried extending it slowly all the way to the pavement (or beyond) and lubing the mechanism?
Judging just from what you told us and your address, Michigan, I'm going to assume you have had the trailer several years or bought it used and it is several years old and has been pulled on salted winter roads in MI and the mechanism is rusted. You may be able to squirt WD-40 or similar product in it and gently crank it back and forth until you can lower/extend it fully and then clean and lube it. If it is too rusted or seized to extend you may have to grind off the end of the support pad under the tire, remove the tire, then remove the entire hoist and replace it.
Harvey