If you are for some reason unable to log into navyfield, be it your internet has disconected or nf is having server maintenance or what have you... there is no way to exit from the game once you have started it.

the more knowledgeable computer users will know how to alt+tab then ctrl+alt+del to close navyfield, but if you didn't know how to do that then you're forced to power down your pc and restart.