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After more than 13 years, and tens of thousands of questions, Moz Q&A closed on 12th December 2024. Whilst we’re not completely removing the content - many posts will still be possible to view - we have locked both new posts and new replies. More details here.
Before you changed the file back did you see the last time it was modified? might give you a clue what happened. As Alan said check other files have not been changed (malware/hacked) again checking the modified dates gives you a clue ( files you know have not been touched having a recent modified date, all the files having the same modified date... etc)