After reading through the manual page 94-104 I think it might be a problem with the automatic settings. I would most certainly try to adjust the settings before taking it in for service. When your pictures are underexposed, Nikon recommendations are
If sensitivity (ISO equivalency)is under 1600, raise sensitivity.
• Use built-in Speedlight.
• In mode:
S Lower shutter speed
A Choose a larger aperture (smaller f/-number)
Just in case you've lost your manual, it is available here Of course once you have exhausted your option you may have to take it in for service of the flash. I suppose you could try a horseshoe flash as well. Good Luck and let us know if your pictures get illuminated.