Blank screen after changing CPU and RAM

hey , I just bought an athlon proccesor with am3 socket, and a 4gb ddr3 ram. I installed them but the screen is blank when I power the computer. If I install the old 2gb ddr2 it works. What could be the problem?
 
Your motherboard is not compatible with ddr3 memory.? Did you check your motherboards specs before purchasing the memory and CPU? Post the make and model of your motherboard. This way someone help you out.
 
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