How to flatten the photo printed on a Black Styrene board

I have printed 11×17 print on Black Styrene (metallic print) couple of years back. It was put on my office wall, now the office is closing so I brought it back home. I have noticed that over time it has bent a little. Is there a way to flatten it without damaging it. Someone from my office really liked the picture so she is asking if she can have it. So, I want to make sure I give a flat frame rather than a bent one. Please help.