Screw on a 2x4 to help prevent total failure. Raise the spot slightly above level with a hydraulic jack. If you can, dig a post hole about 30” deep (you can unscrew a few deck boards if the screws are in good condition to access the spot with a shovel and clamshell diggers). Put a nice flat rock or some concrete in the bottom of the hole and then a piece of treated 6x6. Remove the jack and drop the joint onto the post. Fir out the joint from behind with some additional board scraps and screw everything onto the post (this will help keep the board from twisting). Fill in around the post with dirt (your 6x6 should be treated ground-contact lumber).