If it is HDPE, it probably deformed from heat rather than moisture. You might be able to bend it back after moderately warming it up, for example in boiling water (certainly works for my Seago oil extractor pump, which is also HDPE and gets all floppy when filled with hot engine oil). Although I suspect it would be tricky to get flat.
Alternatively, if the surface is not important (mine have mirrors on the locker doors covering them entirely) and they're not too badly bent, you could shove it through a planer and live with the curvature on the inside (slowly - the stuff is only machinable if you stay below the melting point, else it turns into horrid gunk that is tricky to clean off your tools).