I had the same problem and it was because my wife left her purse in the car with the key fob in it. Thus the Prius detected that the fob was still in the car and wouldn't allow me to lock the doors via the button.

I had to take my fob out of my pocket and lock it with the button. Too bad the system can't be smart enough to detect the one in my pocket and realize that another was left in the car.