Diagnosis
The door lock mechanism (or latch) isn't engaging. Could be a physical obstruction, a misaligned door, or a faulty lock assembly.
How to Fix It
Ensure the door is fully closed—sometimes a sock or tag gets caught in the seal. Inspect the latch area on the door and the strike plate on the tub for debris. Clean with a damp cloth. Gently press the door closed while trying to start.
If That Didn't Work
The door lock itself might be defective. On the MHW5630HW, it's an electronic lock that can fail. You'll need to test it with a multimeter for continuity or replace the whole unit.
Heads Up
Don't try to force the door open if it's locked during a cycle; there's a manual release usually behind the lower kick panel, but use it only in emergencies to avoid damage.