Don't know what I did, but ...
 
One moment I was editing a vocal track with Melodyne as a region effect in SONAR Pro
 
Next thing I know, that track only opens Loop Construction in Multidock; it won't open Melodyne.
That is, Region FX > Melodyne > Open Editor is now GRAYED OUT (only Create Region Effect is available).
 
Playback is normal, and I can hear the previously done Melodyne edits.
 
A different vocal track opens Melodyne in Multidock just fine (i.e., Open Editor is available), but the one I'm working on will only open Loop Construction in Multidock.
 
Edit: Discovered that Loop Construction kicked on when I press ctrl-z to undo an edit.  Bound that shortcut to Undo in Preferences, but ctrl-z still seems to invoke Loop Construction.
 
Anybody know what I fouled up?  Thanks.

 . . . did you by mistake turn that clip into a groove clip ?

Sure feels like that's exactly what happened.
 
Also just discovered that it happened when I pressed ctrl-z to undo something (already changed that shortcut). 
 
Still need to know how to get it outta that Groove Clip. 

Maybe you accidentally rendered it.  Sounds like it if you prior edits are still there.  If so, can you just "create region effect" again and keep going?

Tried Create Region Effect again and Melodyne appeared to rescan it, losing previous edits and returning track/clip to the raw form -- edits were lost.
 
Discovered that Loop Construction kicked on when I press ctrl-z to undo an edit.  Bound that shortcut to Undo (in Preferences), but ctrl-z still invokes Loop Construction.

<<<Tried Create Region Effect again and Melodyne appeared to rescan it, losing previous edits and returning track/clip to the raw form -- edits were lost.>>
 
Hopefully you saved your earlier version with the edits and can go back to it if you want.

This is a good reason to render Region FX.
What happens is...You move something that is not capable of an Undo (a volume change, pan change, preset change).
Then, you use Undo thinking it will undo that change but instead it removes the region FX.
Remember you must have focus in Melodyne in order to use CNTRL+Z to undo edits within Melodyne.
The second you click back in to the UI and not Melodyne Sonar thinks you are trying to Undo the whole Region FX you created, not each edit you made in Region FX.

To clarify my previous post,
It takes a while to get used to this but begin using good RFX habits.
For example: If you are the type of person who opens Melodyne in the MulitDock then clicks on the timeline, Solo button, exc. Make absolutely certain you put focus back in to Melodyne (by clicking within Melodyne in the Multidock) before using CNTRL+Z. If you do not, you risk Sonar, believing you want to undo the whole Region FX rather than just a simple edit you made.

Excellent response, Chuck. That's really useful info.  Thanks so much.