A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
Are you working in a file first created in version 2003?
If so, this sounds like the still-unacknowledged still-unresolved bug where the Freeform, Scribble, and Curve, drawing tools draw invisible lines (no color / no thickness), and although the line colors can be changed, they cannot be set to a default – it won't set a default at any of those tools. It's a bug. I found it had to do with the accent assigned in the original color scheme does not transfer to the color theme in later versions, so those tools are left without a color assignment, hence invisible. The most assured way to fix it is to create a new presentation and copy it all over. There's several postings about this on the board with no resolve and no one was able to confirm it to be an officially known MS issue, so I'm not confident MS is even working on it.