|You have given us very little to go one. No make/model of motherboard, no mention of the type/speed of the old RAM, no mention of the type/speed of the new RAM, etc.|
It could be the RAM you bought is incompatible. Did you buy a single 2GB stick? Did you check to see if your board supports more than 1GB per slot? Is the new RAM the same voltage as the old? Did you try the new RAM all by it's self to see if it would work?
Try using Crucial's memory advisor or scan tool: