If you have lost your mobile device and it is switched off, you can still find it based on its last known location. If you have an Apple device, you can use the Find My app or use the Find My iPhone service on iCloud. If you have an Android device, you can use Find My Device at www.android.com/find.
Android device users also have other options such as Google Maps and AirDroid Family Locator. Samsung Galaxy users can also use Find My Mobile.