Unfortunately, as far as I can tell, and find, there is no way to turn it on. Some software (if their main app is installed on your phone) will allow you to turn off a phone, lock it, etc. I have even heard of ones that will have it make a loud noise so you can locate it. But, none that I can find that will turn it on so you can find it.
Hope you can find it!
For future reference... I know there are trackers you can buy (blue tooth trackers) and other similar items, that you can install on a phone to keep this from happening. You might want to consider that, once you find the phone.