My Ipod 3rd gen. wont work other than when its pluged into the pc

My Ipod 3rd generation wont work other than when its pluged into my computer/or laptop by the charger. The second that i eject it and unplug it from the charger it instantly turns off. When replug it into the computer it will show the gray screen with the white apple symbol in the middle then automaticly turn into the "connected" screen...after charging it for over 6 hours it still does the same thing(unplug it and automaticly turns off)....ps:after unpluging it i try pressing the buttons like menu and center button at the same time it wont do anything...sorta like its frozen...just shows the black screen like its in sleep mode..yet when pluged, it works just fine and i can listen to all my music and stuff...so i know the keys arnt stuck...please help!