what can i see in this forum, that at least every 2nd person starts thread with panic crying "my laptop won't turn on!" WITHOUT identifying make and model of laptop. and there's 2 definitions of "won't turn on" cases:
1st case. laptop REACTS on pressing power button, but nothing happens with LCD screen, but you can see that power on light turned on and in some cases fans and HDD's started to spin. we, laptop servicemen, call this case as: laptop won't start. that means, that unit can't finish POST procedure completely. reasons can be various: broken BIOS image, dead southbridge, dead memory modules,videochip etc. in this situation, very useful device is the POST-card(mini-pci, or, for new laptops - LPC card). using this card, you can check POST passing(or not

)
2nd case. laptop can't turn on. no reaction on pressing power-on button. again, reasons can be very various(lack of SB voltages,dead KBC-controller,dead BIOS ROM,southbridge,PWM-controlles,etc).
don't forget, that for any laptop model, there are TYPICAL failures. for instance, for IBM T4x series(except t43) the typical failure - is the videochip failure. this happens due to videochip positioning onto motherboard(in the middle), and thus, broken BGA soldering. as this chip is compaunded,so you can only replace it, or replace motherboard itself.
i think, it would be better to stick this post. every person, who wants to describe his problem with laptop should read this.