It could that you have white text on a white background. Does anything show up if you select some items ? Click at the far left to avoid opening the bookmark.

Try to select a theme that has dark text on the tabs (toolbars) if you currently use a theme with light text to see if that fixes your issue.

It is white font on a white background. It was previously black font and I didn't change the font or theme. How do I change the theme or font back for just the sidebar? Thanks.