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I'm new to Blender and 3D modeling in general. I've been working on a model for about 5 hours. Just as I was preparing to render and save a picture of it, parts of the model started disappearing. They're not deleted, and there's a light source, they just don't show up. At first it was only like this when I looked through the camera using Ctrl-0, but now nothing it working like it used to when I started. Oh and I'm using Cycles Render.