It could be the CPU. I've had the same problem before.
You could start by checking to ensure that you have the system speaker connected to the correct pins. If you can at least get to the point where you can hear the beeps, it would make life much easier in trying to diagnose the cause of the problem.
Try disconnecting all non-essential hardware, ensuring you have only the core components connected that your system would need to boot. If it still doesn't work, the likelihood is that one of the items you have left connected is the guilty culprit. If you're lucky enough to have the option available to you, try and swap the offending item out with a spare/borrowed item you KNOW to be working.
If it DOES work, add the CD/DVD rom, any 2nd sticks of memory, 2nd HDD's and any other non-essential devices back again one-by-one until it stops working again to pinpoint what else might be causing the problem.
It's also worthwhile (and I hate mentioning this) that you have everything connected properly, especially power. Refer to the manual if needs be, I guess we all like to think we're pretty technically savvy but I surprise myself sometimes with the basic things I tend to overlook