Honestly if the phone is off (turned off or battery dead) there is nothing you can do to locate it until it's next powered up. And if some one turned it on and if they haven't reset the phone, then you could try an app called Plan B. If they have reset the phone and registered it to a different gmail account, then the only option you have is to report the phone stolen so the ESN/IMEI can be blacklisted. If it is already reported, then it won't work on mobile networks, depending on your location.