when i press the power button, the bottom screen just blinks then shuts down immediately by itself. i've took it apart and reconnected all the mini cables but nothing changes. T_T

I recently talked to a user on another forum with the same issue. The solution was to make absolutely sure the top ribbon cable was firmly connected. He used a tool to push it in farther than his fingers could, which fixed the issue.
It's worth a shot at least. Just don't break your DS while attempting to fix it, obviously.