More information is probably a very good idea.
1) What type of motherboard?
2) What type of RAM? (# of sticks, RAM speed, Brand, etc)
3) Which RAM slots are you using on motherboard?
4) Have you tried moving RAM sticks around into other slots?
5) Have you tried removing the upgraded RAM and reverting to older RAM? Does system now boot?
You could have bad sticks of RAM, you could also have bad slots on your motherboard. You could also have two kinds of RAM in your system that aren't compatible. You could also have RAM in your motherboard that isn't running at the right voltage.