My wooden cutting board bent. Is there a way to fix this?
A couple possible solutions:


Throw it out and start over by buying a better cutting board or getting a non-wood alternative that is easier to use, cleaner, and more "knife friendly".  If you want a wooden cutting board, make sure its made from multiple pieces glued together, and each piece is "straight grained".  (The grain of each piece is nearly straight and perpendicular to the cutting surface.)
Take it to a wood working shop and have it planed down so it is flat again. Depending on how warped it is, and the original thickness of the board, this may be not be feasible.
If you insist on trying to flatten it yourself good luck.  I've done a lot of wood working and its been my experience that once the wood is warped, unless its thick enough to plane it and still keep a usable thickness, you're out of luck.  You're fighting mother nature and the natural tendency of the wood to want to follow its grain pattern.