I realize this is an older thread, but this may be helpful: I just replaced the RAM in my laptop. All went smoothly until I tried to reboot...crickets.
I was reading about the voltage compatibility and other issues described above, so I thought that could be an issue. I took the back off again and something didn't look right - I pushed the stick in deeper (sorry) and rebooted and that fixed it straight away. So always make sure the hardware is installed correctly!
On an unrelated note, when I wanted to find this thread in order to post this response, I searched in this website's search bar for pc won't boot after replacing new ram and then "pc won't boot after replacing new ram" and couldn't find this page. Plugged it into Gurgle and I found it that way. Not good.
Cheers