My walls randomly turned transparent or deleted… and toggling them with ‘K’ (back edges) is not working. Can anyone help ASAP? 
Show us a screenshot of what you are seeing ASAP.
Maybe you enabled Xray?
Change the camera to Perspective and then press Zoom Extents.
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That worked! Can I ask what happened?
Working with the Camera set to Parallel Projection you managed to get the camera a long way from the model. DOn’t do your modeling with the camera set to Parallel Project. Leave it set to Perspective except for when you create scenes for plan and elevation views of the model.
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Thank you thank you thank you!
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