I checked my phone 6S this morning only to find that all my contacts have vanished. Whilst I can restore them from a backup I would like to understand why the contacts vanished in the first place.

Can any member enlighten me please.

Many thanks

Depends how they get to your phone. they maybe synced from your mail account or icloud.
Go to settings - mail, contacts, calendars and have a look at the accounts. check any accounts and see if contacts is enabled. the icloud one is a good place to start.

Thanks good ideas but not resolved - I restored data to my new 6S initially from an iPhone 5S back up via iTunes and did not have icloud activated on my old phone.

Checked out the apple website last night and no help available so have manually added my contacts to resolve. What a pain!! I now have icloud activated on the 6S.

As a matter of interest to all members my old phone was with 02 so I phoned them at 19.30 last night to request an unlock and by 21.30 my phone was unlocked free of charge - pretty good service from O2 especially as I was moving to another provider.