According to information provided by the manufacturer, when the power supply light is red this can typically mean that the XBox 360's external power supply (AC adaptor) has encountered a problem and may need to be repaired or replaced. Most problems encountered are typically limited to an issue where the system has overheated or there is too much voltage/electrical current for the power supply to handle.
I'd first suggest checking to see if the AC adaptor is properly connected to your XBox 360 or is warm to the touch. If the adaptor is hot, you may wish to unplug the adaptor for several hours to cool before reconnecting it to your XBox 360. Even if it is fairly cool, you may still want to see if disconnecting and reconnecting the adaptor resolves the issue. If the power light changes from red to an orange hue once you do, this can mean that the power supply may not have been securely attached and the unit is now in standby mode.
Next, if you have access to a second power adaptor, you may want to see if switching adaptors resolves the issue instead. If the XBox 360 goes into standby mode with a different adaptor plugged in but not the original, then the original adaptor may be defective and be in need of replacement.
Please keep in mind however, that if the lights around the power indicator light are also flashing red this may be a sign of an entirely different hardware problem. If you are seeing any of the lights in the outer ring light up, you may want to take note of which ones (and how many) are lighting up at any given time. Typically one flashing red light in the outer ring may indicate a hard drive problem, two red lights may indicate the XBox 360 itself has overheated, three red lights mean that the unit requires service/repairs, and four red lights mean that your audio-video (AV) cable isn't securely connected.
If you're still not quite sure how to proceed, you can always visit Microsoft's knowledge base website for more information and suggestions by using the link I've provided below:
Hope this helps you out.
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