What have I done wrong?
Only one of my tracks shows all the clips in gray and also I can not edit them (though I can move the notes within PRV).

However Clip Lock Position/Data are disabled for these clips.
Can anyone tell me how to reverse it?

Select it the hit the K key?
 
Mute / Unmute

bitman
Select it the hit the K key?
 
Mute / Unmute

No, they are not muted. 
I just can't edit these clips on track view...

What do you mean by "edit"? Move them? Enable the Inline PRV? Delete them?

Anderton
What do you mean by "edit"? Move them? Enable the Inline PRV? Delete them?

Yes, I can't move or delete them.
It must be something very simple, as in the image above, I must have pressed some key, but I can not reverse it for this track.

What happens if you expand the Track Lanes - any difference?
 

Well, the same way that the problem appeared, now it disappeared: mysteriously.
The track is showing the clips normally again.
I'm getting used to living with these supernatural things ...

Tell your cat not to walk across your keyboard when you're not looking 

Is it possible you accidentally "Archived" the track? I don't think I've ever Archived a MIDI track so I'm not sure what happens to them.
 
Just a thought. Try hitting the Archive button (in a separate project or a Save As) to see if it mimics the behavior you saw earlier and report back if you can just for the sake of future forum/google searchers.
 
Cheers!

Beepster
Is it possible you accidentally "Archived" the track? I don't think I've ever Archived a MIDI track so I'm not sure what happens to them.
 
Just a thought. Try hitting the Archive button (in a separate project or a Save As) to see if it mimics the behavior you saw earlier and report back if you can just for the sake of future forum/google searchers.
 
Cheers!

Thanks, but an Archived track still allows to move or stretch clips...