My engine light on my 2000 Nissan Frontier is on. What might be causing this?

Will it turn off by itself?

Running out of gas in a new model vehicle can cause some issues and turn on that light. Usually it deals with the emissions system. Not sure what the code is. The light will most likely not turn off by itself, but you can reset it. If you feel the truck is still running well now that you have fuel back in it, you can clear the light by unhooking the battery and waiting a while (some people say half hour or so). Though some vehicles have a capacitor that will still hold that information even if the battery is disconnected. I would advise getting a reading on the code (Advance Auto Parts, or stores like that should be able to read the code for you and tell you what it means, or if you have a friend with an OBD II Code Reader). You can use that to clear the code. I don't think this would cause an issue that would require an expensive fix (unless a part was about to fail anyway, and this put it over the edge)