How to how do I reactivate a stolen iPhone found

If you do not remember the Apple ID Password then follow the steps given in this Apple Article -> If you forgot your Apple ID password - Apple Support

If you do not remember the Apple ID then follow the steps given in this Apple Article -> If you forgot your Apple ID - Apple Support

If you are the owner or the owner has shared all information with you then to open Activation Lock:- If it is Activation locked, read here --> Turn off Activation Lock - Apple Support Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Learn how to remove a device from a previous owner's account.

If you are not the owner and do not have the Apple ID and Password then there is no way that you can get into this iPhone unless the original owner shares the Apple ID and password with you. If you are lucky you can get into this iPhone if they (owner) accidentally remove the device from their Apple ID.

Turn off Lost Mode on an iPhone, iPad, iPod touch or Apple Watch

When you find your lost device, do either of the following to turn off Lost Mode:

• Enter your passcode on the device.
• In Find My iPhone on iCloud.com, click All Devices, then select the device that is in Lost Mode. Click Lost Mode, click Stop Lost Mode and then click Stop Lost Mode again.

Note: If you had any cards suspended when you put your device in Lost Mode (credit or debit cards for Apple Pay, student ID cards or Express Travel cards), when you turn off Lost Mode and sign back into iCloud, you can use them again.