Sphero has lost connection to his brain. To jumpstart him we need to run a high powered magnet around his circumference.
Set Sphero on a flat surface and run as high a powered magnet as you can around his circumference (imagine he is the earth and you are running the magnet around the equator). If done successfully his light will go out.
The following video will illustrate the technique http://youtu.be/ah5G9_pJuYo
Pop him in the charger. Take him out. Shake him awake and you should be good to go.
If you have a button on the side of your charging base, place Sphero on the base and hit the button a few times, then allow Sphero to charge for at least an hour. This is will do the same thing as the magnet.