Office 2007 & later store their content in the XML format is zip archives with extensions like docx (Word) & xlsx (Excel). If someone changed the extension to zip, that would cause Windows to treat the files as ordinary zip archives and give access to them as such. In such a scenario, the Office applications wouldn't open the files as their file associations don't include the zip format. If this is what has happened to you, you will need to change the zip archive extensions back to the appropriate docx/xlsx, etc extensions. The Office programs will then be able to open them again.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]