Relationship between entities returns null

I'm having trouble checking a parameter in a relationship between the Historian and Login entities.

public class Historico {
    public int ID { get; set; }
    public virtual Login login{ get; set; }
    public DateTime inicio { get; set; }
    public DateTime final { get; set; }
}

public abstract class Login
    {
        public int ID { get; set; }
        [Required]
        public string Nome { get; set; }
        private string Senha { get; set; }
    }

Because whenever I query the database, the Login field returns null. I would like to know what the modification has to be so I can return the login when accessing with the database.