My computer won't turn on, any ideas?

I have an E machines model T2862, that won't turn on. When I push the power on, absolutely nothing happens. I've tried different sockets, different cords..and still nothing. I did take the shell off to look, and when I plug the power cord into the power supply, you can hear a "power noise" you know, when you can tell something is plugged in? The only thing I notice is that a green light illuminates on the motherboard. This is the only thing that happens. The power on still doesn't respond or illuminate. The weird thing is that there is a watch type battery on the motherboard. Is this something that could go out? Do I have a power supply bad?? I have a laptop for all my important stuff, so the e machine is for internet for the fam, and music storage. Thanks in advance for the advice.