The much-hyped iOS 9.3 update hasn't gone smoothly on older devices (specifically, the iPhone 4S, 5, 5C, and 5S; and the iPad 2, third- and fourth-generation Retina iPads, and the iPad Air), preventing users who forget their Apple ID passwords from activating their devices.
As such, they've pulled the update and in the meantime, they 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. Another option is to downgrade to 9.2.1 by clicking Update or if that fails, Restore, but be warned: this may erase your data.
A new version of the update has been released for the iPad 2, so users should be good to go on that front. The rest of you will be taken care of "in the next few days."