Open the Contacts app and check whether the missing contacts are in a group or account
Go to Settings
Tap your name
Tap iCloud
Turn Contacts off, then turn it back on
Choose Keep on My iPhone if prompted
Go to Settings
Tap Contacts
Tap Accounts
Select your email account and make sure Contacts is enabled
Sign in again to the correct Apple ID if needed
Restore contacts from iCloud.com if they were deleted recently
On a computer, go to iCloud.com
Sign in with your Apple ID
Open Data Recovery
Select Restore Contacts
Choose an archive and confirm restore
Restore from an iPhone backup if needed
Erase the iPhone
Set it up again
Choose Restore from iCloud Backup or Restore from Mac or PC
Check the default account in Settings
Tap Contacts
Tap Default Account
Select the account that contains your contacts
Restart the iPhone
Update iOS to the latest version
Contact Apple Support if contacts still do not appear