You need to update a model in Lumion 8.x but accidentally made a Group of your objects and you cannot undo that.
Before you made that Group, you could see the Object Icons but now they are hidden.
Here's how to access the objects:
In Lumion 8.5, you need to click on this button before you can select a Group:
The Group selection icon can then be selected.
You might need to move the camera further up in the air to see the Group Icon.
If you cannot see the Group Icon, then it could be much further away from the objects and camera.
In version 10 and newer the interface is now different for displaying and selecting Groups: use the Select All button: