Please try the solutions....If one doesn't work, try the next solution.
'''Solution #1'''
Is your game disk dirty? If so, place water (you may use a sink..) on the disk, and use a towel or any other possible materials to get the water off. Proceed by placing the disc inside the Game Cube and see if it works.
'''Solution #2'''
Did you insert a disc inside?
'''Solution #3'''
Unplug everything and re-plug them.
Hope this helps! :)