Check it's not trying to boot from a USB stick or memory card you might have left plugged in, and you've not got anything on the keyboard pressing a button down.
If that's no good, try taking the battery out, press and hold the power button for about 20-30 seconds. Then put the battery back in (and the mians) then see if it will turn on.
Failing THAT, maybe trying taking the RAM out and swapping the sockets over.