You can get the 3D massing back into ArchiCAD ... but you can never get things back as an editable model, with walls as Walls, windows as Windows, etc.
In Artlantis 4.1, File > Export... and save your model as SketchUp, which you can then open in the free version of SketchUp to verify and perhaps clean up.
You'll notice that all ArchiCAD layers that were present in the Artlantis file ARE now present in the SketchUp file. Many textures show up there, too. This can help you bring what is there back into ArchiCAD because...
...the skp import for ArchiCAD brings an entire SketchUp file in as a single, monolithic GDL object.
So... you can turn on just the layers you want - perhaps even one layer at a time, save that from SketchUP ... import it into ArchiCAD ... and at least have a bunch of objects, one per layer, corresponding to the massing of your lost model. Then, re-model each layer as required... Remodeling won't be easy as there will not be snap points.
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