You haven't told us what happened to the navigation app, which makes answering a little hard.
My guess is that you allow Maps to auto-update, and it has updated to version 7. In many cases the Navigation icon disappears when it does this. The Navigation function is still in Maps (choose a location and you should get the option to navigate to it), but the interface is a bit different.
In fact navigation has not been a separate app for a couple of years - the Navigation icon actually just launches Maps in Navigation mode.
If this is the case (and it's easy to tell - open Maps and it will look quite different), and you decide that you can't live with the new way of doing things, you can use the Applications menu, select Maps (in "All") and select "uninstall updates", and that should give you an older version back, including the Navigation icon.
If the problem is something else, please post again with a more detailed description.
Hope this helps