Computer won't turn on

I have a iMac G5 that is currently running 10.4.11 and has a classic enviroment that runs as well. The OS X has been quiting a lot lately but now the machine won't turn on. I have unplugged and replugged in the machine but still nothing. I can hear the machine "humming" when the power cord is plugged in but nothing happens when I hit the power button. Any suggestions or is there any way to force the machine to turn on?