Hi,
Moodle comes with authentication out of the box. You either log in, or you are allowed in as a guest (if so configured), which does not require a username. Unless you are using a non-standard third part authentication plugin, I do not know how you can only authenticate with a username only. To reinstate the requirement for a password just re-enable the default manual authentication. If it is enabled - then check the users' authentication methods.
Moodle comes with authentication out of the box. You either log in, or you are allowed in as a guest (if so configured), which does not require a username. Unless you are using a non-standard third part authentication plugin, I do not know how you can only authenticate with a username only. To reinstate the requirement for a password just re-enable the default manual authentication. If it is enabled - then check the users' authentication methods.