Is this because the memory module is defective or not compatible?

I bought a 1gb DDR2 for my computer its a itx board so its single channel and before it only had 256mb. the memory fits perfectly but when i power on the computer there's nothing on the screen, no beeps any thing but its on coz the fan on the processors is running and when i hit the power on nothing happen so had to unplug it. but when i putt he old 256mb back it worked fine. so is it becsuet he momory is not compatible or its defective by the way the mother board can handle 1gb max