You'be updated to Maps 7. You'll notice that Maps looks different if you open it.
Navigation isn't a separate app, but part of Maps. Most phones lose the navigation shortcut when they update to Maps 7, but it was only ever a shortcut to open Maps in nav mode. So you can still navigate using Maps: select a destination and directions to it and you have the option to navigate.
If you prefer the old one, you can select Maps in the Applications menu (under All) and uninstall updates. This will give you back the Maps the phone came with. You will also have to turn off automatic updating of Maps or it will just go back to Maps 7 (personally I turn off auto updates completely, as I like to check what has changed before allowing stuff to update).