Power LED is on in motherboard, but comp wont turn on

If your power supply has a power switch on the back of it, make sure it is on. It should be when the '-' side is down, and the 'o' side is up. I only mention this because it actually happened to a friend of mine on his first build.

Star checking connections inside the case as well. Make sure everything is connected the right way and what not.

If it just isn't doing anything when you press the power button, The problem is limited to motherboard, cpu, power supply, or the case connections. First thing to check would be the front connections on the case. Make sure you plugged everything into the right pins on the motherboard. You might want to look at the manual for your case/motherboard to make sure they are all on the right pins. I've accidentally plugged the power switch into the power LED pins before...which doesn't work so well...

There should be a spiral wire thingy that connects from your "power" button to the motherboard. Make sure that wire is correctly inserted on the motherboard

it is either the power switch connector or the cpu power connector make sure you have connected this one. It is a 4 or 8 pin connector located usually on the left side of the cpu socket.