Many iPhone users are facing a cruel reality that their iPhone keeps deleting contacts/messages randomly without asking for permission. This is a common situation especially when they upgraded their iPhone to iOS 11.
Actually, you don't need to worry so much, if you want to recover these deleted contacts & messages, read through this post and you will find a solution suitable for you:
iPhone Deleted Contacts And Messages Automatically, How to Recover?


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Well, you can recover them from your iPhone backups , like iTunes and iCloud, but it just doesn't support us to restore them selectively, so we'll need to restore the whole folder.

For iTunes:
launching your iTunes, connect your iPhone with PC. when it's recognized, click iPhone image icon.
go to Files--Summary, then you can see all your backups files on left side bar, choose the one you need, click "Restore Backup", and confirm with "Restore".
For iCloud:
Connect your iPhone with Wifi, log in iCloud with your Apple ID and password.
Then go to Setting--General--Erase All Content and Settings, when all content and settings are erased, go to set up assistant with a hello screen.
swipe to begin the set up progress, choose backup files you need, tap on "Restore Backup", and confirm with "Restore".

also when you have no backups, you can come to iRefone for help. it's a professional iOS data recovery software.
for detailed steps, you can check below article for reference:

Get Back only Favorite Contacts on iPhone 7
Try iPhone Contacts+SMS Recovery tool to help you recover deleted contacts and messages from your iPhone. Actually, no matter there is a backup or not, you can easily get them back with its help. It will scan out your iPhone and display them on program window. Then we can easily choose them to recover.
It's really easy to do that. If you have backup file, you can restore your iPhone with your backup file easily and get back all the deleted or lost contacts and messages. However, the new data on your iPhone will be wiped. If you don't have backup file or don't want to restore your iPhone, you can use iPhone Data Recovery program. It can scan your iPhone deeply and find the deleted contacts and messages on it. Learn how to retrieve deleted text messages on iPhone here: Besides messages and contacts, you can also recover deleted photos, videos, call history, notes, reminders, calendars, voice memos, voicemail, safari bookmarks, Whatsapp messages and other deleted data from iPhone to computer.
First of all, you should verify whether you have iTunes or iCloud backup to save iPhone contacts and messages. If you have an iTunes or iCloud backup, congratulations. You can chance to recover contacts and messages from backup files. What should note is that it will easily erase other data if you directly restore data from backup. The more reliable way is to take advantage of an iPhone Data Recovery.

The 2 articles will be helpful to you:
iPhone contacts Disappeared? How to Recover Deleted contacts from iPhone
How to Recover Deleted Text messages on iPhone Without Backup