Q: Does anyone know what to do when there is a white light flashing on Apple TV? It says to hook to music note. Help

It means the update failed and your AppleTV can't start.

You need to restore by connecting to a computer running current iTunes via a USB to micro USB cable.

See:

http://support.apple.com/kb/HT4367

Pay attention to differences in power connection depending on whether or not you have AppleTV2 or AppleTV 3.