This issue popped up over the weekend only on one user's computer. Updates are all current.
In the stencil pane, all 'Lines' are invisible. They all look fine when they are moved over into the working sheet, just not in the stencil pane. The right pic is how it normally looks.
This is to do with Themes. Your user has selected a Theme where shapes have no outline. Some time ago Visio changed the way it shows its stencils to mirror how they may look when the shapes is dropped on the page. Select a different type of theme and you will find the stencil appearance will change.
Ribbon File -> Options -> General
and ensure 'Enable Live Preview in Shapes Window' is ticked
