Check if they are compatible, check if they are correctly placed, check if they fit properly the slots, take out the dust (very important, might fuck up the contacts or whatever), if anything check if one of the ram cards isn't working at all or a slot.
I just went through a RAM pain in the ass, my screen wouldn't even show up, tell us if you hear a long beep when your computer turns on and you can't see your screen, that means one of the mems isn't placed correctly or doesn't work, search on Google to find out about more "beep" messages, they are helpful if you actually get the "beeps".
EDIT: Woops ignore my post.
Last edited by Alan; Jan 7, 2012 at 05:08 AM.