Check this article If your Mac is lost or stolen - Apple Support
After you locate your Mac, you can unlock it with your passcode. Make sure to use the passcode that you created when you locked it with Find My Mac, not your device passcode.
Here's how to see the passcode that you used to lock your Mac:
- Sign in to icloud.com/find on a Mac or PC.
- Select your Mac from the Devices menu.
- Select Unlock. After you follow the steps to verify your identity, you'll see the passcode that you need to unlock your Mac.
The passcode is valid for a limited time. If you forget your passcode or can't verify your identity to see it, take your Mac to an Apple Authorized Service Provider along with your proof of purchase.