I tried the same thing this morning and piano roll turns grey but not the track.
 
This has happened to me twice yesterday.  Both times i was working on a set of tracks all from the same synth and copying/pasting midi data from one track to another. Nothing happened at that point but only after I went into Piano Roll and then copied/pasted midi data from one channel to another in the channel view at the bottom.  All copy/paste (in track and piano views) were "Blending old with new" type. 
 
The first time it happened, I was working on 8 tracks with track 1 being my baseline and the copy/paste on tracks 2-8.  Tracks 2-8 were greyed (but unlocked using the add/delete synth thing).  The second time it happened, I was working on 8 tracks (different ones) with track 1 being my baseline and the copy/paste on tracks 2-8.  This time tracks 3-8 were greyed.  That one threw me for a loop because I couldn't reproduce it exactly as the first time.
 
The only thing I can think of that I did differently is that I have Autosave turned on at 10 min intervals.  Maybe had something to do with an autosave interrupting something?  I know that's a shot in the dark but really the only thing I can think of that may have been different between the 2 times it happened (other than two different sets of tracks but same procedure).

Although this seems illogical (and actually a bug), my solution in post #25 to include (or delete) a new plugin has worked fine, to work around this problem.
Try downloading the sample file that I put in this post, to simulate the situation.
 
 

Had the same problem today, just find out with luck I had to change, in the MIDI track controls, the "Edit Filter control" back to the value "CLIP". I think it happened while I was working with some lane automation. The "Edit filter control" value was changed to the "automation lane name" instead of the value "CLIP".

About "Edit Filter control" see Cakewalk.com  On-line Documentation  subject : Changing track settings
go to    Figure 78. MIDI track controls. check for  C. Edit Filter

I've been struggling with the same problem. Somehow the Shift-thing worked out for me! Thank you!  

KaiRMusic
I've been struggling with the same problem. Somehow the Shift-thing worked out for me! Thank you!  

I tried again with the same file I mentioned in post #25 and this shift-click does not work, and I've already upgraded several times, I'm currently with the 2017.09 version. 

I was having the same problem and as CarriageHouse suggested, holding SHIFT and clicking the grayed out track solved the track problem for me.
 
 
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Eric Lieber 
eric.liebersoftware.com
 

rogeriodec
For those who want to do the simulation, here is the link of the project, mentioned in the images (I apologize, it has more than 10 instances of Kontakt, I could not remove just to keep the symptom):
https://drive.google.com/file/d/0B81wA4UHCedmVzFfMWNDTER4d0E/view?usp=sharing

Rogeriodec, can I ask you what version of Kontakt you are using? I am seeing similar issues to you on a project that is using the latest updates to Kontakt and Sonar Platinum in that the clips in a track will turn grey without me being aware that I have done anything to cause this. I don't recall it happening prior to the latest Sonar release.