HELP! Track editing mutes all other tracks.

This is the second time of posting this here. By the afternoon, when I got mod approval, it had vanished!

The problem is that recently, when I edit a track it mutes all other tracks.
Additionally, my midi keyboard has stopped playing through Cubase and now only plays when I am editing a track and not at any other time.

Is there a link here? What have I done?

Many thanks in anticipation.