How to transfer songs onto ipod touch 4g?

oh my god. can someone please help me. i have a huge problem with my ipod touch. for some reason when i was syncing my ipod touch to my computer, (wifi was off) all my music somehow tragically all my songs disappeared. then when i tried to drag the songs from my desktop to my ipod, it did not let me!!! i even tried going to the music tab on itunes for the ipod but it won't let me sync music from the desktop. PLEASE HELP. REWARD FOR BEST ANSWER.