If you suddenly see this it’s because your the keys you use for ssh are either gone or not know to ssh. If you have them, but they just aren’t registered:

$ ssh-add -l (The agent has no identities.)
$ cd ~/.ssh (to see your keys)
$ ssh-add ~/.ssh/id_rsa (or whatever your key is called)

Also, make sure macOS (10.12+) uses your SSH key’s passphrase from the Keychain automatically

$ chmod 600 ~/.ssh/config # temporarily grant write access to the config
$ echo "Host *\n  UseKeychain yes\n  AddKeysToAgent yes" >> ~/.ssh/config
$ chmod 400 ~/.ssh/config