Don't know whether this could be your problem but same thing happened to me just recently.
I've been using my Compaq laptop w/ac power and battery installed for a long time had no problems whatsoever. The other day tried to turn it on and...nada, nothing happened, just a dark screen. Power was getting to it because the green light was on on the ac adaptor box and on the ac indicator on my laptop. Didn't know what was going on and I thought I'd have to spend $130 - $180 for another lithium battery (argh).
Called Compaq technician and explained the situation. He recommended I remove the battery and unplug the adaptor. Said he didn't want any type of power supply on the laptop. Then he told me to hold down the power switch for 30 seconds and he timed it. With a puzzled frown, I did as he recommended even though nothing happened. Then he had me replug the ac adaptor, make sure it was snugly in place and try to turn it on. Miracles of miracles, it booted up. He then had me insert the battery and unplug the ac adaptor. The notebook detected the battery, prompted me to charge the battery and within 2 seconds the laptop shut off. He said all I needed to do then was recharge the battery.
He did explain that this sometimes happens because laptops have dual power supplies and sometimes the bios system gets confused; at least something to that effect?? So, by holding the power switch down for 30 seconds it completely drains the internal power supply. Not sure if it only happens to Compaq laptops. But I was pleased with Compaq's technical service and more delighted that my laptop was up and running again. |-0