My Acer Aspire 4920 does not boot

Hello,

My Acer Aspire 4920 cannot boot. The LED light is on, but when I power it on, the screen is blank. The harddisk doesn't spin. The fan spinned for 2 seconds and then stopped. I have just replaced the RAM with a new 1gb DDR2, and the situation is still the same.

You guys have any idea? I think it comes from the motherboard :(

Please help :( Thank you