Steam it. Try using an old steam iron on the highest/hottest setting you can obtain. Wet the wood with a spray bottle and sponge. When the wood is fairly wet (not sopping) place the piece on something very, very flat - such as your tablesaw. With a sheet, an old towel or some paper under it move over the wood slowly, but not so slowly that you risk scorching the wood.
When it is flat put wooden cauls (ones that will not stain the wood) on top of it and weight them down - I like to use my tool chests but anything heavy will do. You are likely going to need to do this several times, each time getting closer. If so, flip the piece and work on alternate sides. Patience is key - don't get frustrated and try to get it all back at once. If it comes back you will need to wait several weeks for it to properly season again. Best of luck ....