Assuming this isn't a joke, do you mean the oil filler cap? Most likely way it could "pop off" is if it wasn't put back on properly (not tightened past the detents). Quite possible if you had your oil changed at a quick change place and the attendant wasn't paying attention (maybe it never got put back on at all?). I would temporarily cover the hole (duct tape?), get a new cap pronto (auto parts store, Ebay), and change oil after you get the new cap installed.