no audio from sessions

[crushed]

Sometimes a session won't output any audio. I can see the audio in the playlists which look fine but no sound and no green on the LED.

I've checked my I/O routings and they are all fine.
These are sessions which have always worked fine for months.

the only thing I've done recently is update from OS 8.6 to OS 9.
I'm using Protools 5.01 on a B/W G3 with plenty of memory allocated.

Any ideas?
Thanks.