No display after upgrading RAM

I had two 512 MB sticks of RAM in a Gateway laptop running Vista. I tried replacing them with two 2 GB sticks of RAM. When I did this, I started the computer and there was no display. I took one of the sticks of 2 GB out and replaced it with one of the 512 MB sticks and it worked fine. I checked and it was recognizing both sticks. I shut down and replaced the 2 GB stick with the other 2 GB stick to make sure they were not faulty and that 2 GB stick seemed to work also.

My question is why do I get no display when using both 2 GB sticks together?