Greyed out notes in piano roll

HELP!

I have created a track with 7 regions. The track WAS fine but now, all the notes are greyed out in the piano roll.

I can see all of the notes and the info for each seems intact (position, length, pitch) but there is no playback of these notes.

If I copy the regions into another track, the notes remain grey.

If I copy the regions into a track on a new project, the notes remain grey.

If I create an empty midi region and paste copies of the greyed notes into it, the greyed notes remain.

If I select the greyed notes and SHIFT+M, the grey notes remain.

If I change the instrument plugin, the notes remain grey

If I 'CREATE NOTE' in one of the 'grey' regions, that note plays perfectly.

I guess I have inadvertently made a change to some parameter, but cannot find what it is.

I would be extremely grateful if anyone can help.

Thank you (as I go quietly mad!)