Diagnosis
The door latch mechanism is likely gunked up with grease and food debris. The moving parts around the handle and the strike plate on the oven body get coated, making everything sticky.
How to Fix It
Unplug the oven and let it cool. Use a degreaser spray or rubbing alcohol on a cotton swab and work it into the latch mechanism on the door itself and the metal hook it catches on. Work the handle open and closed repeatedly to loosen the gunk, then wipe everything dry.
If That Didn't Work
The door might be slightly misaligned. Check if the door is sitting square in its frame. Loosen (but don't remove) the screws on the hinge brackets on the side of the oven, gently re-position the door, and re-tighten.
Heads Up
Forcing or slamming the door can damage the micro-switch behind the latch that tells the oven the door is closed. If that breaks, the oven won't start at all, thinking the door is open.