After the upgrade to Big Sur 11.1, I can no longer access my files in Microsoft Word. How can I do that?

Microsoft does not provide free upgrades between major versions of their Office products (unless you subscribe to Microsoft 365) and older versions of Office are not compatible with macOS Catalina or Big Sur.

If you have a version of Microsoft Office that is not compatible, you can either subscribe to Microsoft 365 or you can make a one-time purchase. See: https://products.office.com/en-us/buy/compare-microsoft-office-products-for-mac?ms.officeurl=mac-compare-redir

Or use the free Microsoft Office on the web: https://products.office.com/en-us/free-office-online-for-the-web

or Google Docs: https://www.google.com/docs/about/

or the free and very good Libre Office https://www.libreoffice.org/

or Apple’s Pages and Numbers

What system did you upgrade from? Beginning with Catalina, 10.15, only 64 bit apps will work with the systems. If you had a version of MS Office 2011 or earlier they will not be supported in Catalina and beyond.

You can get MS Office 23016 and update it to the latest update and it will run in Big Sur. MS Offie 2019, a single purchase item, will also work as will the subscription version MS Office 365.

There are alternatives that can open, edit and save MS Office documents in the Office format or their own format: the free, open-source LibreOffice and Apple's Pages, Numbers and Keynote. LibreOffice is more compatible with MS Office that Apple's apps. But both will work.

TextEdit and Pages can open Word documents. LibreOffice can also open them. Some formatting won't be exact.

You can license a copy of Microsoft Word that works on Big Sur, Office 2019 or a Office 365 subscription.