Check out this screenshot:
Why is everything missing in the sidebar? Normally I would see Desktop, Downloads, a custom list of favorite folders, Applications, Library, etc. Suddenly this started happening and it's super annoying because I have to browse everywhere manually on the right side. This is the Save As dialog in Chrome, but it's the same way in a regular Finder window. I'm in OS X Mountain Lion.
Best Answer
You probably accidentally hid the favourites list. If you hover your mouse to the right of the Favorites text, you'll see a small Show text appear. Click it, and your favourites should reappear.