I have the same laptop, with the same issues, now there are a wide variety of fixes given to you already, that deal with issues with similar symptoms. It is certainly not the HD, as it will post without it! If all you are seeing is the main power light, and are hearing no beeps, as well as these other answers not helping. Try this, take the ram out, then start the laptop, see if it beeps, if it does not, and nothing else has worked, your motherboard needs to be re-flowed.
The issue lies where the video card connects to the motherboard, over time, the solder will become porous, and present a bad connection. Its actually a common issue with this model, i have had to re-flow my motherboard 3 times now, now i never turn it off, it only goes dead when i turn it off, and it cools down. The constant fluctuation of hot and cold, causes stress fractures in any metal, but add that dynamic to the fact that flowing electrons do product friction, and cheap solder will become porous. Re-flowing heats up the motherboard, since solder melts at such a low temperature, it will liquify without damaging the motherboard, (if you do it right) allowing the solder to re-solidify once it cools.