“Word found unreadable content”

You open a document and Word says “Word found unreadable content in … Do you want to recover the contents of this document?”, or “The file cannot be opened because there are problems with the contents.” Usually most of your writing is still inside the file.

What a .docx file really is

A .docx is a zip folder in disguise. Inside are small files: one holds your text (word/document.xml), others hold styles, pictures, headers and a table of contents that tells Word where everything is. Word refuses the whole document if any one of these is broken, even when your text is fine.

What usually broke it

The save was cut short. The computer crashed, the laptop went flat, or a USB stick was pulled out while saving. The end of the file is missing, and the table of contents lives at the very end.
A sync or download went wrong. OneDrive, Dropbox, email or a browser download stopped part-way, or wrote zeros over part of the file.
Bad characters in the text. Text pasted from another program, or a document made by another app, can contain characters Word's own format doesn't allow.

Free things to try first

Click Yes when Word offers to recover the contents. It often works for small problems.
In Word: File → Open → Browse, select the file, click the arrow next to Open and choose Open and Repair.
Look for an earlier version: right-click the file → Properties → Previous Versions, or the version history in OneDrive or SharePoint.
Check for an AutoRecover copy: File → Info → Manage Document → Recover Unsaved Documents.
Try opening it in LibreOffice (free), which is more forgiving than Word.

When those fail

Word's repair gives up when the file's table of contents is gone or the compressed text is damaged part-way through. Office File Repair reads the file piece by piece instead. It recovers every part it can find, repairs damaged text up to the last complete paragraph, picks up again after the damage where possible, and builds a fresh document that opens normally.

You see a preview of what came back before paying anything. It runs in your browser, so the document never leaves your computer.

What can't be recovered

If the file is 0 KB, or all zeros, the writing was never saved to disk and no tool can bring it back. Try the AutoRecover and Previous Versions steps above instead.

Check your file: the preview is free