contacts disappear on iPhone 11

Contacts are “hosted” on the server that provides your email, then synced to your phone. So if your email isn’t working, or you deleted an email account, that would result in the loss of contacts maintained by that email account.

If the contacts were specifically hosted on iCloud then see→iCloud: Restore contacts, calendars, and more in iCloud Settings. Note that this will only work for iCloud hosted contacts.