Is my hard disk bust?

Hi,

After a power cut, my Imac failed to reboot correctly. It took an eternity to start up and I ended up on the disk utility page. I ran a diagnosis an the system appeared to be ok.

I was able to get onto my wi-fi system but unable to either install a new copy of OS X, or to restore from my back-up external disk. In both cases there was no target disk, i.e.. my main internal hard disk.

I presume that this means the disk is failed, but don't understand how I can get as far as the disk utility.

Any ideas or confirmation of the diagnosis?