It's in the top posts in this forum.
After that you try it without the HDD, CD/DVD for a test as well as with no RAM.
Usually it's the motherboard if the generic reset doesn't help.
My 12 yr old sons laptop shut itself off and now won't power on. I've removed the battery, used just the AC adapter and still nothing. The power light will come on for 10-15 seconds and then goes back off but it never attempts to power on. PLEASE HELP!