I assume you have also tried another headset to rule out the possibility that your current set or earbuds are not damaged or faulty in some way?
If so, you likely have a hardware issue with the device. A restore to factory settings would have corrected any sort of software glitch or error caused something such as corrupted data.
You'll want to either take or send your iPod in for service/repair.
I have tried other ear buds, and they worked in a other speaker unit ( connecting the iPod to a set of speakers ) but it doesn't have audio when I turn it on - it plays, and I can feel some vibrations, but no audio.
is it worthwhile to try and fix ?????
it's about 4 years old, and I only use it sporadically ..
kinda disappointed it does not work