Owlet doesn’t store artwork with the scene, it stores file paths to images instead. So if for some reasons the image file is not available at the stored path, Owlet will not be able to load textures.
You can fix this by clicking Tools->Resouce Manager in the main menu. The resource manager window will pop up:
The window lists all the external resources of the current scene (mostly images, but can also include ior-files) and marks each resource with a status flag at the left. The green mark means that the resource has been found and loaded well. The red cross means the opposite. The buttons at the top let you fix possible problems with the assets.
Select the resources you need and use one of the buttons above to fix the problems:
The most common issue with resources is that all the images are now located at the different folder. This way you simply need to select all the resources, click Change folder and point Owlet to the new folder, then it will look for the images there and update their status icons accordingly.
If a linked file was renamed, use Change file button instead to show Owlet the new file it must use.
If something goes wrong, you can always use Undo or Reset buttons to revert to the previous or original path. If you don’t need the missing resource anymore, you can use Clear button to completely remove it form the scene.