Gears of war PC help

I was in the mission where u have to lure a blind monster called Berserk to the open and kill her with the Hammer of Dawn.I did the luring thing and took her to an open ground.But the problem now is that I forgo to take the Hammer of dawn.It was replaced by some other gun.I cant go back because the way back is blocked now.The check point was overwritten automatically by the game .Is there anyway to get around this?

Please help me.Or I will have to play all over again.Please.........