Music showing up in iPad but not in my iTunes library.

Hi, I am new to apple products. Infact iPad is my first apple device and I have been using it for just 2 months. So, if my problem sounds naive, please excuse me.

So, the thing is, the music iPad is not showing up in 'On this iPad' in my iTunes when I connect iPad to my Windows PC using iTunes 11. Earlier, I was using a different PC to transfer data and then I shifted to a new PC. For some time, everything on my iPAD was actually showing up in iTunes until I used 'Back up now' shown in Summary tab when I connected my iPad. After the back up was created, nothing is showing up in 'On this iPad', although the music is still in the iPad.

Is there a way this can be fixed?

Also, now if I have to delete all the music on my iPad, I have to delete each song one by one by 'left slide' in Music app or is there some other way?

Thank you.