If you have a backup of the private data folder of Sweet Home 3D where imported stuff and preferences are stored, simply copy back its content. Otherwise I fear it won't be possible, and you simply were unlucky.
If you imported a lot of 3D models, I suggest you to manage them as a Furniture library with the Furniture Library Editor. This will make it easier to create backups.
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Emmanuel Puybaret, Sweet Home 3D creator
