Music just deleted all my own music from the library.

I started Music with the intention of synchronising to the iPhone and suddenly only my purchased music was visible in the library (plus two numbers I imported earlier in the day). All other recordings of my own music had disappeared. The files are all still visible under iTunes Media/Music, so I can restore them to the library but it will take some time. Plus I need to understand what happened in order to avoid it happening again.

Additionally the newly imported numbers didn't replicate on the iPhone.

Is this a familiar problem and if so, what causes it?

Thanks