Since you've checked for any microswitch jamming (assume you've also cleaned the hotshoe) and done settings/mode resets on the camera, I think it's probably a hardware issue.
Googling around, it looks like there is a construction inside Nikon pop-up flashes that can get misaligned/broken so that either the pop-up won't pop up or won't fire when it's popped up, because the proper contact isn't made. A small plastic tab holding tension on one arm tends to break or dislodge. This may not be your problem (could also be a burned out bulb, bad capacitor, etc.) But fixes for the D70 look like they require partial disassembly of the pop-up flash and then either repositioning the tab, or jury-rigging up a replacement with a toothpick and superglue.
Obviously, if you are not good with electronics and diagnosing hardware issues, I'd recommend sending it in for service, rather than risking getting zapped (even pop-up flashes have capacitors) or breaking your camera. But if this is a second body, you own your own soldering equipment and a multimeter, and you love tearing things apart and putting them back together (not uncommon on Stackexchange), maybe these fixes are for you.