The much waited iOS 9.3 update didn’t go well on some older devices, specifically the iPhone 4S, 5, 5C, and 5S; and the iPad 2, third- and fourth-generation Retina iPads, and the iPad Air. After update, users who forget their Apple ID passwords, unable activating their devices.
The problem so widely affected, that Apple have to pulled the update and in the meantime, and advise you try to reset your password or disable the Activation Lock feature via iCloud.com or by connecting your device to a computer with iTunes. If that doesn’t work, users can downgrade to 9.2.1 by clicking Update or if that fails, Restore, but be warned: this may erase your data.
Apple working on this issue and already release a new version of the update for the iPad 2, so iPad users should be good. The rest of devices will be getting their update “in the next few days.”