Is it in your equipment screen, the one you see when you click the wooden sword icon on the top screen? The equipments that you can equip are on there, if you end up selling or discarding them by accident, then i am afraid you will have to start a new game and report this problem to Hayo so that quest/key items/equipments does not get removed in any ways until you create the key/quest items to explore the place they needed to be used on.