If you can‘t activate your iPhone, iPad, or iPod touch after installing iOS 9.3

Updating some iOS devices (iPhone 5s and earlier and iPad Air and earlier) to iOS 9.3 can require entering the Apple ID and password used to set up the device in order to complete the software update. If you can't recall your password or can't sign in, try these steps.

https://support.apple.com/en-gb/HT206203