The contents of Contacts disappeared on my iPhone

Contacts actually “live” on email servers that sync to your iPhone. So if you have recently removed or logged out of an email account, or changed its password, the contacts associated with it would have disappeared.

If these were specifically iCloud contacts, see→iCloud: Restore contacts, calendars, and more in iCloud Settings

No. iOS 18 should not be required. Do you mean the Contacts icon disappeared, or the contents of contacts disappeared? If you tap Phone, then Contacts do you see them?