If your device is lost or stolen, you can turn on Lost Mode (for your iPhone, iPad, iPodtouch, AppleWatch, supported AirPods, or iPhone Wallet with MagSafe), or lock your Mac.
Tap Devices at the bottom of the screen, then tap the name of the lost device.
Below Mark As Lost, tap Activate.
Follow the onscreen instructions, keeping the following in mind:
Passcode: If your iPhone, iPad, iPodtouch, or AppleWatch doesn’t have a passcode, you’re asked to create one now. For a Mac, you must create a numerical passcode, even if you already have a password set up on your Mac. This passcode is distinct from your password and is only used when you mark your device as lost.
Contact information: For iPhone, iPad, iPodtouch, Mac, or AppleWatch, the contact information and message appear on the device’s Lock Screen. For AirPods or iPhone Wallet with MagSafe, the information appears when someone tries to connect with your device.
Tap Activate (for an iPhone, iPad, iPodtouch, AppleWatch, AirPods, or iPhone Wallet with MagSafe) or Lock (for a Mac).
When the device has been marked as lost, you see Activated below the Mark As Lost section. If the device isn’t connected to a network when you mark it as lost, you see Pending until the device goes online again.
See also Mark AirPods as lost in the AirPods User Guide.