Instructions
Check the light on your power supply to see if it's green. If it is, then you know that part of the system is functioning. If it isn't, then there's a problem with your power supply.
Turn off your Xbox. Disconnect every power cord from each plug and every socket. Reconnect them, being sure that the connection is firm. Turn the system back on. If the red lights are still flashing, you still have a problem.
Unplug the system again and let it sit for 24 hours to cool down. When you plug it back in, be sure that it is sitting on its bottom on a hard, cool surface. You should not leave your Xbox 360 on the carpet, as it can overheat and cause system failure. Turn the system back on to see if the red lights still appear.
Call Xbox support at 800-4MY-XBOX to get help with your system. If three red lights are still flashing, then you will have to turn it in for professional help. If you're under warranty, the repair should be free.