imac G5 will not boot up. there is power but the screen is black. About 30 sec after turning it on, the fan comes on loudly.

I have an old imac G5 (non Intel) The screen went blank but the fan turns very loudly. I cannot reboot the computer. whenever I start it I get power and black screen and the fan comes on loudly after about 30 secs. I have tried rebooting pressing varius keys, but no luck. I once got the flashing question mark, but I can't get that either. I'd like to reboot from the disc, but I cannot insert a disc. Any help out there??