After Catalina download, I am no longer able to access Excel. Help! All my spreadsheets are unavailable, unprintable, etc. What can I do?

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

Yes, you have Apple's Numbers installed in your computer. it is a good app and can open Excel files and save as an Excel file if that is needed. Keep in mind that some commands and features are different, however, so there is a little learning curve.

Numbers is free but is significantly different than Excel. If you want something free that is more like Excel and which is also more compatible with Excel files, you may want to also consider LibreOffice: https://www.libreoffice.org/