Food wedged under auger or screen.
Run unit in reverse while loosening the lid.
While running, pour through 1-2 jugs of hot water to 'flush' out any remaining juice or pulp. This will relieve the pressure at the base of the unit and allow the top to be more easily removed.
Stringy vegetables like celery can become wound around the bottom of the auger if not properly trimmed to the correct length or if too much material is fed into the hopper to quickly.