Itunes music was synced with iPhone 4S now it won t

I've had my music on my imac synced with my iphone 4S for almost a year. During a recent routine sync itune decided it would no longer sync my music.

Here are the facts

- "sync music" is checked

- "sync entire music library" is checked

- all songs are checked

- "sync only checked songs and videos" is checked

- When I connect the phone the capacity bar down the bottom of itunes originally shows it full (40 GB of music), at the completion of the sync it drops to approx 100 MB

- only 2 random albums in the entire library are synced.

- I have tried syncing only selected playlists and artists and it does nothing

- I have "deauthorized" and "authorized" my computer.

Any suggestions please??