Wow, thanks for this post! I realize this is like 3 years old and chances are no one will see my response, but damn this just saved my ass.
I was working on a computer for a friend of mine, just software stuff... but I had some RAM I wasn't using lying around around and decided I would see if it would work in his laptop. I took his out and looked at them, they are not compatible, so I put his right back in, didnt even attempt to install the other. Then went to finish software stuff, and it wouldn't power on.
Ive been freaking out the past few hours trying to find the problem as I really didnt want to have to buy him a new laptop... especially considering I wasnt getting paid for this job in the first place. Turns out the problem was simple.... (2) Always a possibility - You may have: (a) not fully inserted the Ram, or (b) you damaged the a memory module or a component on the MB. Try One stick of memory at a time to see if you can get the laptop to power on.
One of the sticks were not seated properly. The other one was good, just one of them was off a little. Honestly a bit surprised that it did not still power on and just not read the stick, thats what I would have expected, and what I have seen in the past so never considered that was the problem. In this case, the computer acted as if it were getting no power. I was seriously beginning to think I had somehow damaged the motherboard.