During powering on, the screen goes grey, Says loading, but nothing?

After a software update, the iMac rebooted and hasn't come on since. It's been 6 wks now. I've tried unplugging and completely disconnecting everthing. Power button on and it acts like it is booting up, but then it goes to a grey apple screen with "loading" at the bottom. It stays that way forever! I also have a DVD in the computer and the "eject" button will not eject it. This is the first Mac I've owned and haven't a clue what to do with it.....any sugguestions???