Toshiba nb200 not powering up

I have a 5-year old toshiba NB200 not powering up. Power and battery indicator light up whenever i insert the ac adapter but once i press the power button all the indicators will turn off. When i press again the power button no light indicator comes out. I have to unplug and plug again before the power and battery indicator light up again. Action taken are the ff:
1. Checked ac adapter with multitester OK.
2. Replaced RAM with known good stick.
3. Removed the battery and Hold down power button for 1 min without ac adapter. Reinserted battery and plugin ac adapter.

What could be the possible problem of this kind of error? Thank you