failure to reboot--please help!

Ok. I have been trying to get more memory for my aged emac by using an external hard drive. I have successfully cloned it but despite following directions in carcon copy cloner and with OWC tech support I cannot get it to be my bootdrive. I also discovered my pRAM battery needed to be changed. Did that and repeated whole cloning process after I used onyx for a system cleanup and disk permissions repair, just in case. Then I wiped the harddive using erase and install off the leopard install dvd. That went fine and did it's automatic restart. That worked fine. Last night I used software update and restarted to install updates. during the update install after it had restarted the power went out for about 30 seconds. The restart had reached the screen which said something like "installing updates" with the pogress bar. It powered off completely. Now it will come on and shows the pale gray screen with the dark gray apple and goes no further. I have tried turning it off and on at the power button to no avail. Also tried to get it to open dvd rom to try to get install disk in, but it won't let me open it. Please help, I really need to get this thing back up and running as my kids do their school work on it and they have to finish by 5/25/12. Thanks!