Apple Music suddenly not syncing across devices

I have a subscription to Apple Music and without touching any settings my library and playlists on my computer have stopped syncing to my iPhone. I have done all the general suggestions signing out and back in etc and it has not been fixed and now a major portion of my library has exclamation marks on the left side meaning file cannot be found. This has frustrated me to tears. How do I fix this and what has caused it. I have a HP computer that is slow and also my internet service provider is stingy and slows internet speed. Are these factors or is it an Apple Music issue?