as you already changed the battery, headphone and back cover, it seems like you know what to do to fix it. First make sure that the hard drive cable is seated properly on both ends!
Most common when the sad face icon appears is, that the hard drive is corrupted/damaged.
But, you should try this first. Follow the link and set the iPod to disk mode, then hook it up to a computer and restore it via iTunes.
Set the iPod to disk mode sometimes needs a couple of tries, once it took me half an hour.
If it can´t be restored it means, the hard drive is damaged OR cannot be read. That means, the hard drive itself is damaged OR the hard drive cable has been flexed one time to often during repair.
As the flex cable is very cheap in terms of a new hard drive, i would give it a try and buy a new flex cable, before i would buy a new hard drive.