DIAGNOSIS The door latch mechanism is jammed with grease or food debris. This is very common.
FIX Unplug the unit. Look at the hook on the inside of the door and the corresponding latch slot on the main body. Clean both thoroughly with a cotton swab and a degreaser like simple green. Work the door open and closed manually to loosen any stuck grime.
IF THAT DIDN'T WORK The plastic door latch hook or the spring-loaded latch assembly inside the main body is broken. The entire door assembly likely needs replacement.
HEADS UP Do not tape the door shut. The safety interlock prevents operation if the door isn't sensed as closed. Tape bypasses this.