Ipod nano problem

So I plugged my nano into my friends speaker, and something came on the screen that said i had to plug in into my USB cable again. So i plugged it in, and reseted y nano and everything worked fine, i can hear my music and everything, BUT now when i plug in my Nano back into my I-Book the computer doesnt recognize it. I mean the ipod keeps charging but i cant find it on itunes and theres no icon on the desktop showing that its connected to the USB cable. Does someone know whats wrong??? how do i can my Ibook to recognize the Nano again???? please help.