Open the Contacts app and check if the contacts are already synced
Go to Settings
Tap your name
Tap iCloud
Turn on Contacts
If prompted, choose Merge
Go to Settings
Tap Contacts
Tap Accounts
Select your email account
Turn on Contacts
Sign in again if required
Open iCloud.com on a browser
Sign in with your Apple ID
Open Account Settings
Restore Contacts
Choose an archive and confirm restore
Restore your iPhone from an iCloud backup
Restore your iPhone from a computer backup
Check the Recently Deleted folder in your email or contacts service if available
Make sure you are using the correct Apple ID
Restart your iPhone after restoring contacts