One possibility is that the flash isn't charging. I've seen this on my DS in two instances. One was because a circuit card went bad. The other was caused by dirty battery terminals (allowed enough voltage/current for the camera to start but not enough for the increased load during flash charging).
Does the battery indicator on the top LCD go flaky at the same time? Mine would go from full to empty when I tried to activate the flash. This shows that the voltage drops under high current draw.
Try the easiest first - take a pencil eraser and gently clean the battery terminals. If this doesn't clear it up, you may have to send it in for repair. The repair will take a while if a circuit card is bad. The parts have to be shipped from Japan.