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When you click SAVE on a document, the current changes are written to the same file.
So ....if you open a document, make a single change, and then click SAVE, the change gets written to the same file.
The SAVE AS button is what you need to use to keep the OVERWRITTEN phase from happening.
When you open a file, make a change, and then click the SAVE AS button a NEW file is created, and now you have BOTH files. The original document, and the newly created document that has the change on it.
So, if you have been working this whole time with the SAVE button, then yoru original file has been overwritten many times, and the original is now lost.