Quit all apps from Multitasking.
(Double tap the home button to bring up the multitasking dock. Tap and hold the app you want to quit. A red badge with a minus symbol will appear on the icon. Tap the minus symbol to quit.)
Press and hold the Sleep/Wake button and the Home button together for at least ten seconds, until the Apple logo appears.
When done set up the default account in settings and try again.