What causer the engine to lock up.

I recently had an oil change, and after not driving the truck for two weeks because I was out of town on business, when I return I started the vehicle and it started smoking, with the smell of oil. I checked the oil and it was good. Upon driving it, it completely stop. A mechanic checked it out and said it was the timing belt, and that the motor had also locked up. Could that possibly be the problem or something else.