So, as I said before it could be a lot of things and it looks like it might be complicated.
- First, you need to disassembly your laptop, you could do it by following this guide for battery replacement: [guide|107055|guide].
- Check if the motherboard if contaminated by dirt and if so clean it up, if the problem persist...
- Try powering up your laptop with the battery disconnected, in order of determine if your battery is in bad shape.
- If the problem continues, you´ll need to test with a multi-meter if you have voltage in the board, and what happens with that voltage when you press the power button. It could be a component in the main-board that is creating a short once it gets voltage.