iPhone back up via iCloud has not brought back full iTunes music library

Hi,

I had my iPhone 6 stolen and got a replacement. I restored the new one from an iCloud back up of the old iPhone. However, the only music it has brought across has been my iTunes purchases, and not my complete library (of which I have thousands of songs).

I've tried a few things but it's like iTunes is not recognising my old library, only the stuff I have bought with them.

Does anyone have any ideas as to how I can get the rest of my music back?

Any help much appreciated!

Thanks,
Simon