Power Mac g5 blinks 3 times and shows no image on monitor. Any ideas?

My son received a Power Mac G5 as a gift. We tried turning it on and there's no activity on the monitor screen. Both the tower and the monitor lights blink in patterns of 3. The fan on the tower ramps up and gets louder and louder. I'm a serious computer novice and have no idea how to proceed. Anyone have an idea?