This article explains what to do if you have an iCloud-locked iPhone, which means that Activation Lock is turned on as an anti-theft measure. To unlock and use the phone, you will need to enter the original Apple ID username and password. Instructions apply to devices running iOS 7 and later.

Activation Lock is an effective anti-theft measure. Your iPhone or iPad automatically turns on Activation Lock when Find My iPhone is active. Once you turn on Activation Lock, no one will be able to remove the device, activate it on a different account, or turn off Find My iPhone without entering the Apple ID username and password that the phone was originally set up with. If you experience this problem, here's what you can do:

To determine if your iPhone is iCloud locked, look at the Activation Lock screen.

Enter the Apple ID username and password you used to activate your phone.