My iPhone 6s is in Activation Lock. What can I do?

The best solution is to use account recovery. That way you get your account back too. If you do not, or cannot, do account recovery, you can ask Apple to reset the device. This will give you use of the device again with a new account but won't get the old account working. For Apple to reset you need the original receipt. A box and serial number are highly unlikely to convince them the device is truly yours. You can bet that thieves have tried every story possible to try to get Apple to unlock devices for them.

Read this article about removing Activation Lock --> How to remove Activation Lock - Apple Support Unless you have the account information that set the lock, the original receipt from Apple or an Apple approved vendor, or can demonstrate legal inheritance from a deceased owner, you cannot remove the lock; return or recycle the device.

I am always surprised when people don't know their email address. To me it would be like not knowing your name. I use my email address about 10 times per day.

This article tells you the various ways you can try to discover your forgotten Apple ID --> If you forgot your Apple Account primary email address or phone number - Apple Support Typically an Apple ID is an email address you have used. If you are currently using a mobile device that is logged into your account, go to Settings and tap your name at the top. Look in Keychain, and in a browser's security settings if you use it to automatically sign into Apple when using a browser.

This article outlines what to do If you forgot your Apple ID password --> If you forgot your Apple Account password - Apple Support

or go directly to this link --> http://iforgot.apple.com/

If you find you can’t sign in, reset your password, or receive verification codes, you can request account recovery (read this support article: How to use account recovery when you can’t reset your Apple Account password - Apple Support ) to regain access to your account. Account recovery might take a few days or longer, depending on the specific account information you can provide to verify your identity. More detail in —> Activation Lockout Issues - Apple Community

If you still need help this page provides country-specific Apple Support contact information ➞ Contact Apple for support and service - Apple Support