Hi i had lost my mac and now i found it again the guy which found it before me has put the wrong passcode and now i can try again what should i do?

If you can't unlock your Mac

After you locate your Mac, you can unlock it by entering the passcode that you created when you locked it with Find My.

If you can't remember that passcode, you have three options:

• Sign in to iCloud.com/find, then select your Mac from the Devices menu. Select Unlock and follow the steps to verify your identity.
• Unlock your Mac from your iPhone, iPad, iPod touch, or Apple Watch using your device passcode.
• Take your Mac to an Apple Authorized Service Provider along with your proof of purchase.

When the device was lost - had you Locked the Device as per the article from Apple ?

Refer to below

Lost or stolen Device