Ledsteplin
Ambassador
SOLUTION:
These faults are actually really common and here's the solution...
1. Go to SETTINGS:GENERAL:RESTRICTIONS.
You'll probably find that they're turned off. Well, turn them ON! Punch in your pin code and ensure that everything is selected with blue so that they are not restricted.
2. You should now be able to update your iOS AND log in to your Apple ID. Everything should start working again as usual.
This worked in my 4S when it lost Siri and the App Store. It also worked my my iPad Air when I had the same problems signing into. Y Apple ID etc and lost YouTube and couldn't update.
Thread is 4 years old.