Chosen Solution
One of my friends recently updated his iPhone 7 to iOS 11, something strange has happened. He had lost the contacts. He cannot see all of them (maybe 20/100) under the phone book. Is it lost or hidden, so many issues with iOS update! Can someone help please. Cheers
He should try taping the group button at top right corner and choose all available options
Did he backup to iTunes or iCloud before installing the update? If yes, then he should able to restore contacts from the backup. If not, then there are various data recovery tools that can help him get his lost data back.
The best way to install an iOS update is via iTunes. Check this thread Update your iPhone, iPad, or iPod touch - Apple Support
