You've updated to Maps 7.
Navigation wasn't a separate app, but a shortcut to starting Maps 6 in Navigation mode. With the update to Maps 7 most people lose this shortcut. You can still get navigation via Maps: enter your destination, select directions then select navigate.
If you prefer the old one, you'll need to go into the apps menu, all apps, select Maps and Uninstall Updates. That will get you back to whatever Maps version the phone originally came with. If you do this you should turn off auto updates, or else it will just update to Maps 7 again.