Microsoft Word cannot Open

Solution 1:

I can think of two things:

1. Try to open the document in Word with the option to repair it. In Word 2007 you have an arrow next to the Open button where you can select Open and repair. Word 2010 probably has the same.
2. You can try to rename .docx to .zip and then open /word/document.xml (the new Office XML files are just zip-files). Maybe you can fix the problem in the xml file yourself. Make a copy first!

Solution 2:

Apart from the things The_Fox mentioned, I would try opening the file in OpenOffice (or LibreOffice... same thing, really). It might have more of a tolerance for imperfect files.

I would also make a backup of the file, so that it can't get any more damaged/modified.

Solution 3:

The Docx converter of Google seems to be quite fault tolerant.

Perform the following steps:

1. Upload the file to Google Drive
2. Convert it to Google Docs format
3. Open it with Google Docs
4. Finally store it as Word Docx file.

This has just worked almost flawlessly for me!

Note:
Meanwhile, Microsoft published a fix for the "missing end tag" error. For other file corruptions, usage of third-party Word converters can be the salvation.