If you did not coat the back/bottom side as well as the top with your finish, there is your problem. The top gets baked and soaked, but not the bottom.
I'm not sure if there is a cure, because the uppermost surface has shrunk and the lower portion has expanded to create the "cup". You could unscrew it and turn it over and then..... clamp it on the table with a spacer in the center to "reverse" the cup ..... bring it into a humidity controlled environment and see if will return to flat..... clamping with curved cauls may help... do not store it directly on a concrete surface, keep it elevated at least 24" above.
It's a bit of a crap shoot at this point. :frown2:
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)