Find my iPhone
Use Apple's Find My app or Find Devices on iCloud.com with the Apple Account connected to your missing iPhone. You can view an available location, play a sound or mark the device as lost.
What to do
Check that you can sign in to the Apple Account used on the iPhone.
Open the official Find My route and select the missing device; if theft is plausible, mark it as lost before trying to retrieve it.
If it’s not that simple
The device is not in the expected list
Wrong account?
Check the Apple Account that was linked to the iPhone before loss.
Find My was never enabled?
A website cannot enable it retrospectively on a missing device.
Device appears but no fresh position?
Inspect the time/status and available lost-mode controls; a blank map is not proof it was erased.
Good to know
Find My must have been set up beforehand; location can be unavailable or approximate.
Official source
Apple Support · locate a lost device