b) A cheap-looking thing to try, if the notebook opens far enough to select cells, is to select all the cells (or all the Input cells), copy them, and paste them into a new notebook.
If the problem is invisible characters, that might help.
Using Edit menu - Copy As - Plain Text would be better, in terms of leaving accidental junk behind. Outputs, especially graphics, might need to be rebuilt.
c) The attached notebook has a notebook de-corrupter that might be helpful.
The symptoms are unfamiliar to me, so I am offering multiple suggestions.
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