Imac G5....HD not working?

Ok so I'm new to macs and I'm trying to fig out if my sister's Imac G5 HD is not working. I dunno to much about her mac but I do know is a G5 with iSight and her HD is 250GB. When she goes and turns on her mac it goes through the whole apple logo with the little time icon (not sure what's called) then it give her a small window saying loading Mac os x. This is where it gets stuck at. We tried restarting a few time and same thing. Any ideas on what the issue might be? Some1 told my it might be the HD that's shot.

Thanks guys i really appreciate the help. BTW I did do a search and I couldn't find anything. All I found were topics on macs being stuck on the blue screen with timer.

Jose

Pop in the install DVD and reboot, immediately holding down the 'C' key and see if that will load. If successful, go to utilities in the Menu Bar, select Disk Utility and run Repair Disk and see what is reported. It will either pass, repair or advised fail in red.

Does help if you provide machine details, model , processor, operating system etc.