If your placed objects have disappeared, the following may be the cause:
Collected by Imps
If you placed building materials, furniture, observation beacons, or other items in or around an imp encampment, they may have been stored in a chest within the encampment by imps.
Please check the nearby encampment's chest.
Collected by Other Users
If your world settings allow other users to place or collect items, it is possible that another user has collected your items.
Destroyed by Fire
If you placed items near campfires, furnaces, or similar, they may have been destroyed by fire.