Alienware 17: 4 Beeps Error

For quite some time, I have had problems with my Alienware 17 laptop shutting down randomly, so I opened it up yesterday to see if I could find what was causing it. I removed every part very carefully, and placed everything back in the way they were, but now when I start the laptop, it shows a black screen, and plays a series of 4 beeps, which I know is a ram error. I tried testing one ram at a time, but I still got the error with all four. I put the ram sticks into another laptop, and they work fine, so I don't know what is causing this. Is it possible that my motherboard is damaged, or could it be something else? Thanks for your help.