This rarely happens to stock consoles, but I think you might be dealing with RTCBug. If so, the fix is about as straightforward as it gets.
# Power off the console
# Take out the game card (leave the sd in)
# Lay it down somewhere, turn it on, and leave it alone for 15 minutes. If it boots by then, you're all set.
If that didn't work, it's time to check for f000000b corruption. Turn the DS on, wait 30 seconds, and press POWER. If the shutdown menu appears (grey screen with the Power Off button), you'll need to install boot9strap via ntrboot to fix this. Otherwise, next option.
If neither of those work, we'll need to start looking for hardware damage. How far did you disassemble the thing? Knowing which parts you took off should give us a starting point