! in the
shadow entry's encrypted password field means that no password can authenticate against it. From
If the password field contains some string that is not a valid
result of crypt(3), for instance ! or *, the user will not be able
to use a unix password to log in (but the user may log in the
system by other means).
As the manual says, this does not mean that you can't login as root. It just means that you can't login as root using a password for the root account. (You can login as root via SSH using SSH keys, for example, if you had configured it earlier, even if the account is locked.)
sudo normally authenticates with your password, not root's. This can be changed by setting one of
sudoers. If you set one these options, and try to use a password when the password is locked, that will fail.