You must have moved it to Storage. Please open the last tab of the Storage.
Note: you can place objects from Storage only on Big Land or Love Island. While on one of these Islands, go to Storage ->the very last tab and tap Place under an object you want to move out from Storage.
If you are missing a water well, please remember that there are different types of wells. They all have a limited amount of times they give tools. Once the limit is reached, the well turns golden - and that's the last time you can get tools from it. Then it disappears.