f5 no longer brings up sidebar

this problem has arisen. I have reinstalled but still does not work. I have relied on this feature for many years to navigate through chapters in large documents. Scrolling is not an option and remembering chapter titles is not feasible. Any suggestions please.

Try F11.

Operation system and LibO version, please?

F5 opens navigator for me. I’m on Windows 10 and have tried it with LibreOffice 6.0 and 5.4. Please look in menu View. The Navigator and its associated short cut key should be listed there.