Help! Ipod Classic Suddenly Showing No Songs

I had 13,000 songs on an old classic ipod--uploaded years ago with MP3s. Had plugged it into itunes and it seems all has disappeared (shows 0 songs). Is it recoverable? I don't think the songs were in i-tunes to begin with and aren't there now (unless they are hidden).

If I need to start again from my own database of tunes, what's the best way to do it? Thank you for your help. I am itunes/ipod i-gnorant. -B