MDF is a building material consisting of small wood particles compressed under high pressure. MDF or "medium density fiberboard" is used in cabinets, stair-steps, furniture or anywhere a strong perfectly consistent building material is needed. MDF is tough, but not indestructible. Due to its lack of grain patterns, it can crack in any direction if it is abused or too much weight is put up on it. It can be fixed with some glue and clamps.
Pry down on one side of the crack with your hand. When the crack opens up, stick the end of a putty knife in it.
Place the tip of a glue bottle against the crack and hold it tight. Squeeze the glue bottle firmly to force glue down into the crack.
Drag the tip of the glue bottle along the crack as you inject glue into the length of the crack. Remove the bottle.
Smear and force the glue down into the crack with your finger. Pull out the putty knife.
Stretch bar clamps across the MDF and tighten. Wipe off any residual wet glue with a damp cloth.
If the MDF is in a location where you can't put on clamps, the glue alone should work fine. When you pull out the putty knife, the MDF comes together providing adequate pressure for the glue to work. MDL is "medium density laminate;" it can also crack. Forcing glue into any crack of MDL will work the same way.