I caught the corner of my garage with my lawnmower and it ripped the corner post of the siding half way up. How do I repair it

Yep, you can usually just smack them back together. BUT FIRST, use some screws (suggested) or nails (loosen frequently) to make sure each row and the corner post are still fully secured to the building. If smacking doesn't quite get it you can pry the bottom lip with a screwdriver or a store bought Siding Zipper.

- The existing screws or nails can, possibly, be reset but really should be re-installed right next to the old attachment hole. The fasteners should only hold the siding snugly to the wall, but still allow the siding to slide side to side by hand so it doesn't buckle or bow with temperature and Sun exposure changes.