What you need to do is restore it to its original status. Make sure you have the latest version of iTunes installed on your Mac or Windows PC and connect the device, after detection select it in iTunes and if not detected put it in DFU mode. Select the "Summary" tab then "Restore" option and you will be prompted to backup before restoring, select the Restore option when iTunes prompts you. Ones the restore process is over your iPod will restart and the Apple logo will appear while booting up. After the restore you will see the "Connect to Activate" screen and iTunes will detect it.