im suprised no one as asked this yet...
do you know if the power supply is good, as in working and if so is it set to the right voltage, i know mine has dual volts it can run at, it would be a little red switch on the back of the PSU (power supply unit). also make sure there isnt an on switch back there that is turned off... i didnt check that once and i let the computer sit for 2 months untill i realized that switch was off. lol