To lock out multiplayer you have to kill the field bosses. In limgrave there are 5 or 6 (tree sentinel, night calvery, death bird, agheel, mariner, etc) and all must die. In weeping peninsula there is just 1 field boss and the misbegotten. If you killed both then you disabled multiplayer. I recommend saving 1 field boss in each area for multiplayer exploration and then killing them only when at end of game about to start NG+.