First thing to try is tightening the "winch" back up as tight as possible. Sometimes that releases the lock.
If that doesn't work you can put a bottle jack underneath there, lower the winch as much as possible and then use the jack to push up on just the middle part that goes through the tire. Check the manual, it tells you how to do this.
If none of that works you must do what I had to do and take a big long handled screwdriver, get it up through the tire and up to the part that won't come down. You have to get it up there and bang on the end of the screwdriver to release the hook. On the tire carrier there is a hook that is supposed to release when you lower the tire, it gets corroded and doesn't want to function.
Once you swear alot and bust up all your knuckles and get it down, grind off the hooked part so you don't have the lock anymore. Free repair, just a PIA.
Here is a picture I drew, ok I suck at drawing but hopefully you get the idea.
