iMac g5 doesn´t start

So what's wrong?

What seems to be the problem is that The iMac gets stuck on this grey start up screen Then the fan goes crazy as I have a feeling it gets stuck and doesn't know what to do next...

Maybe it needs a fresh install of an operating system but I'm not sure, I don't own an operating system for the computer that I can put on as got the iMac from my old high school. I have had a look inside and all the LED's seem to show that the hardware is working properly .