What motherboard do you have, what RAM did you used to have, and what RAM did you replace it with?
You can also try resetting the CMOS. Unplug the computer from the wall, then unplug the 24-pin connector to the motherboard. Remove the CMOS battery (looks like an oversized watch battery on the motherboard) and leave it out for about 5 minutes. Then pop it back in, plug everything back in and try again.