Ipod classic 160 not visible in tunes 12

Question

My perfectly functioning ipod classic 160 gb is invisible to itunes. It shows on the ipod screen it is connected and it shows on the imac screen as an ipod. In itunes it does not exist. It worked perfectly with itunes before the upgrade. I have seen the device bar and my iphone 5s displays there normally and functions perfectly. When only the ipod is plugged in, there are no devices in the bar and the itunes menu for device is greyed out.

Answer 1

If you have not, disconnect all USB devices (including any hubs).  You can leave standard keyboard/mouse connected, if used.  With iTunes running, connect iPod to a direct USB port by itself.  This is to rule out a problem a conflict between connected USB devices, or a lack of power on the USB bus for all connected USB devices.  If you already tried that, or it does not help...
Since the iPod appears initially in iTunes, the problem may be a "feature" that recent Windows PCs, with Windows 8 and USB 3.0 ports, seem to have.  It is an "energy saver" setting that turns off full power to the USB port if the connected device is deemed "inactive" by Windows.  When you connect the iPod, iTunes does its initial sync for a few seconds.  That's when the iPod is seen in iTunes.  After the sync, iTunes unmounts the iPod's disk.  If Windows considers the iPod to be inactive and turns off full power to the USB port, that is when the iPod disappears from iTunes.
If this is the cause, you may be able to go into Windows Control Panels and turn OFF this feature (so USB ports always stay at full power).  If that does not work, or you can't find such as setting...
Alternately, you can Enable disk use on the iPod.  With disk use enabled, the iPod's disk remains mounted by the system until you Eject the iPod in iTunes, so the port remains at full power.  This may be difficult, if you don't have enough time to click on the iPod in iTunes and check the box for Enable disk use on the iPod Summary settings screen (and click Apply).  I'm assuming you have your iPod set to automatically sync your entire iTunes music library.  If so, you can extend the iPod's initial sync (when you connect it) by making a significant change to your iTunes music library (before you connect it).  If you happen to have some music CDs that you need to import into iTunes, that would work.  Add new songs to your iTunes library, and then connect iPod.  If you use the Sync only checked items setting, you can uncheck a large number of songs in your music library, connect to sync, then recheck the songs, connect to sync.  Making iTunes sync a few dozen songs during the initial sync should give you enough time to go to the iPod's Summary settings screen, and check the box for Enable disk use (and click Apply).
If that does not work (or the iPod still disappears quickly even when there are changes to sync), the cause something else.  FYI - It's not because iTunes 12 does not support discontinued iPods.  I have an 3rd gen "classic" iPod from 2003, and it works fine with iTunes 12.