I have a iMac G5 and my screen is black........ need help!

Sounds like the Energy Saver went on. And if hitting any key doesn't work, and you have a wired keyboard, you may have to give up your file you last worked on and just force reboot it from the power button. These machines have had many power supply and capacitor issues. Getting those capacitors replaced is hard.

try a safe boot & a normal boot. if it only works in safe boot, there is a software fix.

Shutdown your machine. Hold down the shift key. Poweron. Wait awhile Wait awhile while you harddrive

is being checked.

http://support.apple.com/kb/ht1455

the iMac g5's have a mini-vga port. You could connnect some other monitor to the g5.

Robert