Navigation system stopped working

2006 Ridgeline Navigation system stopped working. The screen says "Navigation system door is open or the DVD is not inserted"

1. It used to work, then one day it just stopped
2. Where is the navigation system door?
3. Where does the navi DVD go in, can I check if it is there?
4. Is it possible someone stole my navi DVD?

Thanks in advance,

Ron

The system is under the drivers seat. Eject and reinsert DVD and see if that helps