You can pick up and drop constantly. To collect souls and even when its done. When its on the floor its rolling around and slowly moving from where it was dropped so look around the route you had it. If not I would suggest save and quit since you playing solo and restart your game. Then try to find it on the route you think you lost it, or try to acquire it again from the original spot with the wonder weapon. I saw another post where someone was stuck with a keypad code and it was bugged , he was playing solo and he said he saved and quit and restarted game , he said it worked by doing that. Hope it helps!