iPod classic (160gb) not recognized by computer

I have a 7th gen 160gb iPod classic which isn't recognized by my computer. I recently replaced the hard drive and battery and it seemed to go off without a hitch. After I put the front plate back on the screen lit up and everything was working. I connected it to my computer and synced it to my iTunes library, although it only would only sync 140 songs out of my 6000 song library. I connected my iPod a few more times and eventually the computer stopped recognizing the USB device. I restarted my computer, my iPod, I tried a new USB cable, and I tried plugging it into my other computer. No matter what my iPod wouldn't get recognized. When I plug it in the screen lights up and it charges but my computer doesn't recognize my iPod as a device.