Depending on how warped it is and how strong/thick the wood is, you could try double-sided tape--the kind meant specifically for hanging picture frames.
We had a leak from our instant hot water unit and we didn't discover it for a long time because the cabinet was full. The board is so warped and needs to be replaced ...
See more
Make your own frame and reframe it. It’s very easy to make a frame.
You can add braces on the back.
Both answers are good.
Depending on how warped it is and how strong/thick the wood is, you could try double-sided tape--the kind meant specifically for hanging picture frames.