How to turn off lost mode

Turn off Mark As Lost on a device

When you find your lost device, turn off Mark As Lost.

1. In the Find My app  on your Mac, click Devices.
2. In the Devices list, select the device you marked as lost, then click the Info button  on the map.
3. Under Mark As Lost, click Pending.
4. Click Turn Off Mark As Lost.
5. Click Turn Off to confirm.

You can also turn off Mark As Lost from your iPhone, iPad, iPod touch or Apple Watch by entering the device’s passcode (the one you use to unlock your device).

Note: You can’t turn off Mark As Lost remotely from another Mac. You can only turn it off directly on the Mac that you marked as lost — by entering the numeric passcode you set up when you marked your Mac as lost.

If you forget your device passcode, you can recover it using Find My iPhone on iCloud.com. See Use Lost Mode in Find My iPhone on iCloud.com in the iCloud User Guide.

see section Turn off Lost Mode...

Use Lost Mode in Find My iPhone on iCloud.com - Apple Support