Why can't a user see anything on the backend?
From Joomla! Documentation
This usually happens because a user has been given membership in a group with rights to access the backend but the group has not been assigned to a viewing access level. By default most assets (mainly modules such as the administrator menu and Quick Icons) in the backend are assigned to viewing access level Special. So, most commonly you will want to assign any group with backend rights to the special access level in addition to any other levels it might be assigned.