Q:

Where did my default brushes go?

I was working on a logo design for a client in Illustrator CS4 when suddenly the default brushes were no longer there. The brush files are still in place but they do not show up in the brushes panel. The options to show the brushes by type are checked but greyed out. 
I tried a preference reset, that didn't fix it. I haven't tried any new brushes because the brush I needed was in the stock set. I know its bad form not to try everything but I needed one of the stock brushes to tweak an existing design.

A:

When you open raster images, and sometimes PDF files via Illustrator, the files open without any of the stock libraries. This is because the files don't contain the information about the stock libraries, including the brush library.
It's exceptionally rare for stock brushes to just "disappear". It's far more common for a user to open a file and simply not have the brushes loaded.
If you need the default brushes back there are a couple things you can do.

Select all and Copy. Then open a new document and paste. The new
document will contain the default brush set.

or

From the Brush Panel menu choose Open Brush Library... > Other
   Library... Then navigate to any illustrator file containing the
   brushes and open it. You can even open a new document, save it, then
   use that to navigate to via the Brush Panel and load as a library.

