Suddenly I am unable to enter data onto any of my established spreadsheets. Any attempt is immediately shipped off the working page. How can I restore normal working?
Whatever “shipped off the working page” might mean…
Maybe some key details are needed for your question.
You can click Help > About LibreOffice and press the button next to Version Information to copy to clipboard. Then click edit just below your question to add the info.
In the absence of information, maybe look at answers to Where is info on Locked docs?