First, please make sure that you scroll up and down through your entire backpack looking for the item. On most platforms, the backpack is (or eventually becomes) scrollable because it contains too much stuff to fit on the screen, and people have occasionally not realized that the item ended up in the part of backpack that isn't currently visible.
If you've scrolled up and down through your backpack and the item is definitely missing, it's possible that the items ended up on the ground somewhere (including possibly behind the backpack menu). This is because the items in the backpack can be dragged either to a different slot or to a location on the ground, and the game sometimes doesn't correctly interpret your intent. This will hopefully improve in future updates of the game.
When you go to look for items lost in this manner, note that they may not end up exactly where you think they will. The items I intentionally lost in this manner when studying this issue weren't exactly where I thought they would be on the ground, but they were nearby--within an inch or so. Unfortunately they were sometimes hard to find again (some ended up behind bushes since that's what happened to be nearby), but I always eventually found them. The final location probably also depends on where your finger/cursor happened to be at the point the item was dropped, which won't be the same in all cases. In one case the lost item was a full screen away from where the avatar was standing when the item was lost.
If the item you lost was a tool or "hidden object" quest item and you can't find it yourself, please see this page for something else you can try. These are important, so we built a way for you to get them back if you lose/misplace them. That said, don't intentionally try losing/misplacing your stuff--we may not have anticipated the manner in which it disappeared.