I have D70 and my build in flash stop working. Any idea how to troubleshoot?

I've had all kinds of issues with my flash, most due to my own ignorance or error. Depending on the mode (e.g. auto ISO) the auto-flash will only work if the light is quite low. If you need the flash in this mode you can try manually triggering the flash by pressing the little button left of the flash. Best Shot Selector (BSS) and continuous photos modes will also deactivate the flash. You can always swap out the batteries--flashes need plenty of juice to work.

If these little gotchas aren't your problem there's probably a short somewhere. The camera may require disassembly to find the short. I've heard this problem is not uncommon in the D70. If it was me I'd just take it into the shop (or send it in to Nikon), particularly if my warranty was still good.