Running several times a task?

I have a method button_click which runs a task and another that cancels it. The problem is that when trying to run the task again it throws an exception: 'Start can not be invoked in a task that has been completed.'

This is my code:

private void Iniciar_Click(object sender, EventArgs e)//boton de inicio
    {

        bstate = bstate * (-1);
        Acomodar_cajas();
        ct = ts.Token;
        Task ta;

        if (bstate == 1)//detener
        {

            iniciar.Text = "Iniciar";
            ts.Cancel();

            td_cb.Enabled = true;
            Desbloquear_grupo_compas();
            indice_caja = 0;
            Limpiar_cajas();
            check_box_td_state();
        }
        else//iniciar
        {



            ta = new Task (() => Tarea(), ct);
            ta.Start();
            iniciar.Text = "Detener";
            get_values();//obtiene los valores de las cajas de texto
            bloquear_grupo_td();//bloquea grupo td
            bloquear__grupo_compas();

        }
    }