Cardboard warps if it gets wet or is stored on an uneven surface. Though most times warped cardboard is still usable, it won't lay flat or fold as easily as straight cardboard. It is simple to straighten warped cardboard and doesn't require a lot of effort. With some simple adjustments and a little bit of time, you can straighten your warped cardboard.
Move the cardboard to a dry area with enough room to leave it there undisturbed for at least one night.
Lay the cardboard flat.
Dip your paintbrush in the water and "paint" the top side of the cardboard. If you don't have a paint brush, fill a spray bottle with water and spray the cardboard until it is evenly covered and damp but not soaking wet.
Place a sheet of plywood over the cardboard and weigh it down with weights. These can be heavy rocks, actual weights or any heavy item to press the plywood down.
Let the cardboard dry overnight. Remove the weights and plywood to check on the progress. If the cardboard it still damp, replace the plywood and weights and let dry overnight again.
If it is small piece of cardboard, get it damp, cover it with a water barrier, like cling film or metal foil and place a heavy book on top of it.
The straightened cardboard may not be as sturdy as it originally was.