iPod classic 160 GB says no music after sync

This bit...

Check your iPod with Diagnostics Mode

It is possible that your iPod's hard drive has started to fail. Take your iPod and place your right thumb on the centre SELECT button and your left on the top MENU button. Press down both thumbs for about 6 seconds until your iPod reboots. Immediately move your left thumb around to the rewind button |<< on the left and hold this down together with SELECT for a further 6 seconds. Your iPod should now switch into Diagnostic Boot mode. Press MENU for Manual Test, then select IO > HardDrive > HDSMARTData to reveal your stats. For comparison here are mine taken when my 6th Generation Classic was about 2 years old:

Retracts: 889
Reallocs: 12
Pending Sectors: 0
PowerOn Hours: 2202
Start/Stops: 894
Temp: Current 24c
Temp: Min 10c
Temp: Max 50c

Take a note of your results. When finished press MENU+SELECT for 6 seconds to reset the iPod again.

What are the numbers?

tt2

Hard drives can and do fail without warning sometimes. That said you might want to try restarting your device several times to see if the hard drive will fire up and give you those numbers or the iPod will launch properly showing its content.

tt2

See Re: Corrupt iPod classic. What picture do the hard drive stats tell?

tt2

After doing a Restore, current iTunes sets it to use the Manually manage music and videos setting by default (not automatic syncing). Select the iPod in iTunes (click its device button on horizontal bar) to show its settings screen in iTunes window. Along left side (in the sidebar), under Settings, click Music. The iPod's Music settings screen is shown to the right.

Is the checkbox for Sync Music currently checked? If your complete iTunes music library fits on the iPod, is the option to sync Entire music library selected?

Select the iPod in iTunes to show its settings screen. In the sidebar, under On My Device, click Music. To the right, the iPod's current song list is shown. Does iTunes show that your songs are loaded on the iPod?

So iTunes shows the songs on the iPod, but the iPod's onboard software does not show those songs.

On the iPod Summary settings screen in iTunes, set it to Enable disk use (if it's not already) and Apply. Find the iPod (as a storage device) on the Desktop (or a in a Finder window) and do a Get Info. In the Info window, do Capacity and Available make sense, based on the iPod being loaded with (~18.000) songs?

If the iPod appears to be loaded (as expected) with song files, you may want to review the steps you took previously...

You mentioned using Disk Utility. Did you use Disk Utility to Erase the iPod? When a regular Restore in iTunes does not work for some reason, doing an Erase using Disk Utility (to reformat the iPod's storage) is something you can try. But you need to do it in a particular way. In the Disk Utility sidebar, select the iPod DEVICE, NOT the volume indented below the iPod. Selecting the device makes Erase partition the iPod's storage before reformatting it. Selecting the volume just erases the volume.

If your OS X version is El Capitan 10.11 (you have "10.13" in your profile info), in the "new look" Disk Utility, after selecting iPod in sidebar and clicking Erase, set Scheme to Apple Partition Map (NOT the default GUID Partition Map). Set Format to OS X Extended (Journaled). Name does not matter. Click Erase.

Then, run iTunes and do a Restore on the iPod to install the iPod's onboard software.

I did all that and I got an error message after HDSMARTData.

So strange because it worked fine for years and just stopped working after I added a few songs last night.