In the space of 12 hours I've lost 3/4 of my downloaded songs

[starcollector]

Between the early hours of the morning and now I seem to have lost the majority of music that was available offline in my library.
The songs are still there, but are 'unavailable offline' which is weird as there are a number of songs I listened to yesterday that aren't available today. And yes I do have a premium account.
This has happened before, and I had to re-download everything. It seems ridiculous that I need to do that again.
I only have a small amount of data on my phone, so it's not really possible anyway.
As far as I'm aware I haven't changed anything in my settings on spotify or my phone and service provider.
I'm extremely frustrated as I rely on having music to get me through my days.