This may be due to incorrect placement of the hopper on the base. When set up correctly, the arrow on top of the hopper will point to the arrow on the base. The motor will only turn on when these arrows are aligned.
Another possibility is that a magnet, which activates the juicer's power function, is missing. To check if the magnet is missing, inspect the underside of the lid. If there is a half-moon shaped hollow recess then the magnet is missing and needs to be replaced.