The device may have powered on while you were still working on it, have you done a soft reset on it? Hold power + home buttons down for about 10 seconds, release the buttons and try power it on normally with the power button. This issue can happen alot when swapping screens out. Even if you are 100% sure that it was off when you plugged the screen in last, try the soft reset anyway. This fixes 80% of the iPods that come in after a failed installation by the customer.