See this question: Where did my default brushes go?
The same holds true for Swatches, or any library of items.
To recap, 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 swatch library.
To load the default swatches:
- From the Swatch Panel menu choose
Open Swatch Library... > Default Library... > (Then the color space you want - CMYK or RGB)
This will open a new swatch panel containing the swatches.
- If you want this new panel to go away and have the swatches in the default panel. Highlight all the swatches on the new panel. Then from that new panel menu, simply choose "Add to swatches". You can then close teh new panel which opened.
You can also avoid this by opening a new Illustrator document and using File > Place and then placing a raster image rather than opening the raster image with Illustrator.