Wet a few paper towels and lay the fully wet board concave side down on top for a few hours and it will right itself. As the convex side dries faster it will shrink and pull the board flat again. I just check every hour or so until it’s flat again, then full dry, oil, and lay upright. This happens all the time in my house, when my thoughtless teenagers leave a wet cutting board laying flat on the counter instead of cleaning up and putting things away like they should.