Up until yesterday, all of the songs I downloaded were available offline. This morning, none of them have the green "downloaded" arrow, and they don't play without connection. I have 3gb left on my phone, and they don't appear as downloaded anywhere in my library.
Hi. This can be triggered by various things such as swapping SIM cards, factory rest, not going online for 30 days, third party cache cleaners etc. All you can do is redownload the music which has gone missing.