Do not plug your iPod into the computer! That's the first thing.
There are applications to get the files back off the iPod despite Apple's assurances that it is impossible
I can not find the name of any at present. Googling should work. If I find out I shall post back.
Why the files are lost is a mystery to me and perhaps you should leave the iPod alone until you have solved that. .... were you sharing all the songs you have lost?