I used to manualy manage music on iPhone but now the option to manualy manage music dissapeared. I didn't do anyhthing and I can't add songs without syncing. Help?

So anyway, we all had our share of problems with adding/removing music etc. I used to have the manually manage music option ticked. Now it's not ticked and it's asking of me to sync. I have no idea why this happens. I have never synced on a different pc. This one is the only one I have. Then I though maybe i need to find a different library, and I opened all previous iTunes libraries (to which my surprise all had the same content nothing changed). Anyone know what the **** is happening. These problems with iTunes are srsly making me consider is it worth all the fuss with iTunes. This is not the first time it happened either. Happens often to many users. So does anyone have any idea how to fix this or restore previous library (which is definitely on this pc if antivirus didn't delete it though I don't see how can a library file be corrupt.