My iPod nano won't switch off

I've just bought a nano iPod and for some reason unknown to me it won't switch off. Everytime I hold down the menu button nothing happens, or when something does happen, the apple logo comes on the screen for a few seconds (as it does when you reset the iPod) then it goes back to the 'home' screen on the iPod again. I've tried the '3 R's to sort this out, and since I updated my older iPod as well, that has refused to switch off too. PLEASE help me.