Some of my Contacts have disappeared

iPhone 14. iOS18.6.2. Some, but not all of my contacts aren't visible. Afaik, I haven't domne anything to hide them. In a group chat on Messages, for example, phone numbers show up where contact names used to show up. The messages are still there including photos. If I click on the phone number, it takes me to a detail screen for that person. Again it shows the number and not the name. If I click on "info", it goes to a new screen with the name and offers me the choice create a new contact or add to an existing contact.I can't add to an existing, because it doesn't know of an existing one. If I try to create, it does nothing. I am on the internet most times including now, but sometimes I am not. Any idea what is going on and how I can get my missing contacts back?