What is wrong with my camera? The light turns on and nothing happens, then it goes out again?

The 3 flashing red lights indicate low battery power! This could be because of low battery power (all 4 batteries must be good), the battery contacts in the camera are poor, the make of battery being used (try different make of battery)

Sounds like electrolytic capacitor dried up needs curing, which is power on capacitor and left it absorb some moisture in humidity on power. Some time a few minutes, hard cases may take a few days.

You should reset the camera:
-Hold camera upside down -Open battery/card door -Remove card -Remove battery -Make sure A/C power is DISconnected -You should see a small hole above and to the right of the battery slot, insert a sharp object and press -Re-insert card and battery
If this does not work, try it again a few times, if still unresponsive, the camera may need to be serviced by HP.

Do you have it on viewfinder or lcd screen ?

You probably caused a short circuit when you changed the light. Change fuse #14. Next time, don't turn the lights on until you are finished changing the lights.

If you've only had it 6 months then I would suggest you contact the manufacturer for a warranty replacement. Their warranty period is 1 year so see what they can do.

Set the camera to anything except no flash. Turn it off. Take out the battery. Wait 5-10 seconds. Put the battery back in. The flash should now work until you turn the camera off again (and then you must remove/replace the battery to get the flash working again).

The flash requires some kind of "master reset" to work. Haven't found a permanent solution to this.

Try changing to rechargable batteries. I had exactly the same problem. I used recharable they run out so tried various types of alkaline batteries but the camera would not turn on. Getting the flashing green light. Charged the batteries put them back in and working again

Any Warranty left? If not contact Nikon technical help This is a major fault.

You have probably dropped the camera. If you are brave enough to open the camera up you can put the rollers back on track. Not a difficult problem to solve.