Firstly it is always important to wear a seat belt while driving. Now getting back to the answer of the question, we can say that the person will be thrusted forward like a projectile. Since the car is coming to a sudden halt from a speed, so the inertia of motion will throw the person forward. The person will feel the force equivalent to the speed of the car and the weight of his body. This multiplied force will be of greater hazard for the person that is not wearing the seat belt. In such cases the inertia will try to pull the body out of the seat and force it forward with the multiplied thrust.