No, it cannot. I called and spoke to my Proshares rep.
The only way for it to go to $0, is if the index falls 50% in a day.
The proshares are not based on a fixed point, but reset daily.
If uyg is $10 today. If index goes up 5%, uyg goes up to $11. The follow day, the percentage is based off of the closing price. It is not fixed like the early ETN's were with oil up and down at a base of $50.