Sometimes when an iOS device goes missing, Find My iPhone helps to locate and recover the device. Ideally, the battery on the lost iPhone has enough juice for iCloud to track the location live.
In some cases, the lost device has a dead battery or is simply turned off. So does Find My iPhone still work? Live tracking will not work, but Find My iPhone may still be able to lead you to your missing phone by showing you its last known location.
Instead of a green circle on a map identifying the iPhone's current location, the last known location of the iOS device will be displayed by a grey circle.