If you turned on "Find My iPhone" option on your lost iPhone, maybe you can find back the iPhone. Just follow below steps.
1. Use the Find My iPhone app on another iPhone, iPad, or iPod touch. Or you can sign in to icloud.com/find
on a Mac or PC.
2. Tap the "My Devices" button at the top-left corner of the window on an iPad or iPhone. Select your lost iPhone from the list of devices (If you have other devices associated with your Apple ID, such as an iPad, it will be listed here as well). Or click the Devices menu at the top of the page if you are using the iCloud website.
If your iPhone cannot be located because it is turned off or out of network, check the "Notify me when found" box. As soon as the iPhone is turned back on and connected to a cellular or wireless network, you will receive a notification at your Apple ID email address.
3. If your iPhone is online and "Find My iPhone" can locate it, the map will center on your iPhone's last-reported location. You can use this map to determine where you may have left it.
4. If your iPhone is lost nearby, you can click or tap the "Play Sound
" button to make the iPhone play a notification sound. This will be played at maximum volume, which can hopefully help you track it down. Tap "OK" on your iPhone to stop the sound from playing.
5. You can also choose "Lock Mode
" to lock the iPhone's screen with either your preset passcode or a new one if you don't have one set. You can also add a contact number and message that will be displayed on the screen if a nice person finds
6. If the iPhone is gone and you don't think you'll be able to recover it, you can choose "Erase iPhone
" to wipe your iPhone of all your data. This is a last-resort option, and should only be chosen if you are worried sensitive or personal information will be revealed. If your iPhone is passcode-locked, your information is not retrievable.
For more details, check: How to Find Your Lost iPhone