I would get a jiffy pole and jack it up . also remove any weight on top if possible. the then jack it up as straight as possible. the glue and screw with 3/4 inch plywood and 1 1/2 inch screws with PLR LePage PL Premium Polyurethane Construction Adhesive, Interior/Exterior, Waterproof, 295 ml
and then let it sit for a weekend, the slowly remove the jiffy pole . it it stayed true then fine in not then repeat with a second layer on top of the first . this time with 3 inch screws.