Contents of address book have been deleted

The Contacts on the iPhone can be managed in multiple ways, but they are all email accounts. You are either using iCloud, or another email account. Have you signed out of or deleted an email account. If you check your email accounts, is the Email setting turned on or off? Contacts cannot just disappear without any reason. You say the numbers are still there but just the names are missing? Try a Force Restart on the iPhone and see if that helps. Force restart iPhone - Apple Support

Let me ask, are you referring to the Contacts app on the iPhone, or something else. You say Address Book, I want to make sure you are referring to the same thing.

Given that you are experiencing a problem with Contacts and Calendar, I would suggest checking to make sure that you are signed into iCloud and have Contacts and Calendar both turned on. Go into Settings>[your name] then tap iCloud. At the top you should see your name, that will show you are logged in. Then scroll down just a little and you will see the section Saved to iCloud. You probably won't see everything there, but you should see the command See All just to the right, tap on it. Make sure you have iCloud calendar on, then also that Contacts is turned on as well. If they are, if you did the Force Restart I suggested earlier and it did not help, then try the steps in this support document. Restore your calendars and events on iCloud.com - Apple Support