How should I go about repairing my truck?

Today, I was driving to a friend house. While on the freeway, my truck starts deceasing in speed. I notice that my rpm gauge drop to 0 when I try to go faster. I pull over on the side of the freeway, the truck was still running, I turn off the truck, try to restart (hoping it work like pc's), it didnt start. My problem is, what is it that has happened to make my truck act this way and how can I fix it? Also, I just put gas in it.
-2002 Nissan xterra V6