i had some 80 songs in my MBP and some 800 songs in my iPod, i synced my iPod and now i have just 80 songs in there and thats it. I lost all other songs that were previously in my iPod!
If they are not on the iPod or on the computer they ae gone. You can redownload iTunes purchases by:
You can only sync with one iTunes library. If you sync with another library/computer the iPod will be erased and its media replaced with what is in the iTunes library of the computer. I suspect that is what happened.