the battery icon on my ipod classic 20gb is showing not charging from power supply

i was given an ipod classic 20gb which i connected to my computer. I was then advised to upgrade the operating system to the latest which i did. I was told to connect the ipod to an external power supply to complete the process.

So now the ipod is plugged into the power supply but nothing is happening, the Large battery icon in frozen and not showing that it is charging and i keep being shown a little picture of a plug and socket. Any ideas anyone ??