org.hibernate.TransactionException: Transaction not successfully started
    at org.hibernate.engine.transaction.spi.AbstractTransactionImpl.rollback(AbstractTransactionImpl.java:202).

private Session session = null;
    private SessionFactory sessionFactory = null;

    public UsuarioDAO() {
        sessionFactory = HibernateUtil.getFactory();
    }

    public Usuarios autenticarUsuario(Usuarios usu) {
        Usuarios usuario = Singleton.getUsuarios();
        String hql = "FROM Usuarios WHERE emausu = :emausu AND senusu = :senusu";

        try {
            session = sessionFactory.getCurrentSession();
            session.beginTransaction();

            usuario = (Usuarios) session.createQuery(hql)
                    .setParameter("emausu", usu.getEmausu())
                    .setParameter("senusu", usu.getSenusu())
                    .uniqueResult();

            if (!session.getTransaction().wasCommitted()) {
                session.getTransaction().commit();
            }
        } catch (RuntimeException erro) {
            if (session.getTransaction() != null) {
                session.getTransaction().rollback();
            }
            throw erro;
        }
        return usuario;
    }

<hibernate-configuration>
    <session-factory>
        <!--  Configurações de Conexão ao Banco de Dados  -->
        <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/meuBanco?zeroDateTimeBehavior=convertToNull</property>
        <property name="hibernate.connection.username">root</property>
        <property name="hibernate.connection.password">minhaSenha</property>


        <!--  **** Propriedades do C3P0 - Pool de Conexão ***  -->
        <property name="hibernate.c3p0.min_size">1</property>
        <property name="hibernate.c3p0.max_size">50</property>
        <property name="hibernate.c3p0.timeout">1800</property>
        <property name="hibernate.c3p0.max_statements">50</property>


        <!--  ****SQL Dialect ****  -->
        <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>


        <!--  ****Gerenciamento do Contexto da Sessão****  -->
        <property name="current_session_context_class">thread</property>


        <!--  ****Desabilitando Cache de segundo nível****  -->
        <property name="cache.provider_class">org.hibernate.cache.internal.NoCacheProvider</property>


        <!--  Mapeamento das Entidades  -->
        Minhas entidades

    </session-factory>
</hibernate-configuration>