Remove the ceiling tile from the grid by pushing it up above the grid. Turn the tile and pull it through the grid vertically at a slant. Be careful not to damage the tile by removing it from the grid.
Lay the bent tile on a hard, flat surface with the concave side facing the surface.
Mist the ceiling tile with water using a spray bottle. Do not soak the the tile, but just dampen it slightly.
Apply heat to the tile using a handheld hair dryer. Move the hair dryer over the entire surface of the tile at a good distance in a back-and-forth motion for several minutes.
Push the tile gently against the flat surface. Use the combination of the spray bottle misting and the hair dryer to loosen the tile so it is more flexible. Keep gently trying to manipulate the tile until it is laying flat on the surface. Allow the tile to dry completely before replacing it into the grid -- overnight is best.
Replace the tile back into the grid by inserting it through the space vertically at a slant and then laying it horizontally in the grid once it is above it.