Click “Expand” on right sidebar.
Right sidebar extends by a small amount (e.g. 1/4 width of screen) and shows excerpts of text from other notes containing linked or unlinked mentions of currently active pane.
It seems that as some point you made the right sidebar huge and now it covers everything else. You can resize it to make it smaller
Hover over the border and pull it to the right. Be careful not to select the left sidebar.
Ah brilliant, that was an easy fix, thank you!