For Microsoft Word “.docx” documents with Zotero or Mendeley citation fields that were inserted with the Word plugins from these reference managers, you can use Reference Extractor.

If you still have the references in a reference manager, you can export them from this program to a file format for exchanging bibliographic metadata, such as RIS or BibTeX, and then import this file into Zotero.

You can follow these steps to format the bibliography as BibTeX, which Zotero can import:

1. Download this Word bibliography stylesheet.
2. Save the stylesheet to Word's bibliography styles folder:
   • Word 2016/2019/Office 365 for Windows:

     C:\Users\<currentusername>\AppData\Roaming\Microsoft\Bibliography\Style

   • Word 2010 for Windows:

     C:\Program Files (x86)\Microsoft Office\<Office version>\Bibliography\Style

     or

     <winword.exe directory>\Bibliography\Style

   • Mac: Go to the Applications folder. Right-click on Microsoft Word and choose “Show Package Contents”. Navigate to:

     Content/Resources/Style

3. In Word, change your bibliography style to “BibTeX export” and copy the bibliography to the clipboard.
4. Use Zotero's Import from Clipboard function.

If the references have ISBNs, DOIs, or PubMed IDs, you can use the Add Item by Identifier function in Zotero to quickly add these items to your Zotero library.

If you have many references, you can use AnyStyle, ​an online bibliography ​parser written by a Zotero developer. Export parsed citations as BibTeX or CSL-JSON and import them into Zotero.

Otherwise, your best option is to find the items online in a repository that Zotero supports, or, as a last resort, manually enter the references.