How to navigate at night after a complete navigation systems failure?
This is no different then flying during daylight. Your first choice is GPS ( for most people :)). Failing that you fall back on radio navigation ( VOR ). Failing that you fall back on your charts and NavLog. NavLog is essentially pilotage and you rely upon your airplane speed and direction and checking waypoints along the way. If you chose your waypoints carefully ( on the ground when creating your NavLog, I use airports so that I can land of them in case of an emergency) and you know how they look like from air ( I take satellite photos ), assuming that your waypoints are 10-15 minutes apart and you took care of wind speed and direction in your navlog, there is really a minimal chance of getting lost.

Ocean is a different matter. Maybe you will need to look at stars or something. In VFR we are told not to fly over the water, day or night.