In most cases you can straighten a curved board as it is nailed into place. Wet lumber is usually flexible enough to straighten with hand pressure. If hand pressure will not work you may need to try using pipe clamps or a special board-bending tool but you will need a solid surface to fasten to. Br careful if using pry bars to try to straighten a curved board. This may end up damaging the board. In severe cases you can try to use a jig by temporarily fastening a 2x4 brace about 8’ alongside the curved board then use a 2x6 or 2x8 wedge to drive between the brace and the board to straighten it and then fasten it in place. If nothing else works you may need to cut the board in order to salvage pieces of it.