OK been there done that (twice). It is all too easy to delete files to your recycle bin. The problem here is if you have a relatively full hard drive there might not be enough room in there to hold all the deleted songs.
I have learnt my lesson, the "do you want to send files to the recycle bin" button is very easy to press, especially if you are not concentrating or a bit sleepy.
Do yourself a favour, turn off all iTunes assistance, turn off syncing and manually manage your music. you will not regret it.
Keep the iTunes music folder (the default in iTunes) totally empty and load your music from another folder. That way iTunes can never ever delete your music again. iTunes (when sync'd) also assumes that because you want to delete music from your iPod that you also want to delete it from your library and hard drive, totally crazy in my book.
Message was edited by: Stubchain