My built-in flash D50 is broken. It is popping up, but it isn't flashing. It fell down, so is it broken, or can I still fix it?
Unless you're a camera technician, it's broken beyond self-repair.
Don't mess around with flashes if you're not skilled with high-voltage electronics and camera internals. Flashes need to store charges of 300V or so in order to work, and may not discharge all the way when you turn the camera off. If you open up your camera and touch the wrong thing, you'll get a nasty shock, and could end up dead.
If the risk of death isn't enough to scare you, add in the risk of frying or otherwise breaking the rest of your precious camera. I'd really recommend taking/sending it to a repair shop. Chances are, you'll need to replace some parts, and a shop will either have them on hand or be able to get them quickly.