As soon as you feel that the elevator is falling, immediately lie on your back on the floor of the elevator. In this position, the strength of the blow will be spread equally across the area of your body.
In this situation, the best thing you can do is sit on the floor of the elevator. In this position, the load on your skeleton will be less than if you were standing. If there’s very little space, then at least try to bend your knees, as this will also help to reduce the force of the impact.