Syncing between iTunes and iPods has always been one-way, you cannot simply copy music back from an iPod to iTunes. There are programmes for recovering music from an iPod and putting it back into iTunes, although I've never tried them and I believe they all cost money (after a free trail period). Check this tip, from user tt2, that has more information about recovery software: https://discussions.apple.com/docs/DOC-3991
I have also read that Window Media Player can rename or move files, which will cause issues with iTunes, although I have never seen any evidence of this myself.
So - if your missing songs are no longer on your hard drive (not even in the recycle bin) and you have no back up, your only options are to obtain them again from your original source (possibly at additional cost to you) or try the software mentioned in tt2's article.
Other methods to replace your lost music:
- music from CDs, that you no longer have - buy the albums again
- music purchased as digital files - most (but not all) stores will allow you to download previous purchases again. You do of course, need to sign into your account with that store
- music downloaded from artist websites etc. - go back to that site and re-download if possible
Although you won't thank me for pointing this out now, you will do in the future: back up your Library!
- Music on portable devices such as iPods and iPhones is not a backup
- Portable devices get lost, stolen or dropped
- As you have already discovered, Apple devices are not able to copy back from the device into iTunes
- The hard drive on your computer can fail
- I read recently of someone who believed they had a backup of their Library, only to realise that they had their "backup" on the same hard drive on their computer
So make a backup of your existing Library, before that too gets lost. Buy yourself an external hard drive and copy your music to that drive. You can use a bit of software called SyncToy to copy from one drive to the other (just search for SyncToy online).