iTunes shows no music on device

I don't know whose suggestions you followed, but normally the only time that a Sync would empty out your phone is if the phone was being synchronised with a different iTunes lIbrary to the previous one (i.e. a different computer, or on a machine that had been re-built or repaired). In either of those circumstances, iTunes gives a warning that if you continue with the Sync, it will remove all the existing content from your device.

Even then, that would not cause all the music in your iTunes Library to disappear.

So let's try and establish what happened and when.

1. Before you connected the phone to iTunes, was the music in your iTunes Library and did it play?
2. Is that music still in your Library? If not, is it listed as being there but doesn't play, or is the Library apparently empty?
3. Did the music on the phone come from a Sync with this specific iTunes library on this specific computer? If so, has the computer been repaired or re-built?

Once we have your answers, we can hopefully help you recover from this situation.

Which device, is it an Apple product?

Have you set Sync options for it in this specific iTunes? (Each device is separate, if you have set Sync options for another Apple device, it will not apply to this device.)