That would depend on a number of factors, but in all likelihood they would probably wait until another wave of attackers moved forward and join with them. For reasons that are obvious retreating across No Man's Land after being repulsed was almost as dangerous as attacking across it. If no more attacks were forthcoming then attempting to move under the cover of darkness or some other distraction would be best. Snipers, machine gunners, and artillery posed significant threats to moving targets on No Man's Land. For that reason if an attack failed you likely found whatever cover could be had and waited for some type of relief or cover to withdraw.