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.
- 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.