I have the latest Melodyne Studio 4. I usually render any Melodyne edits I do to a clip by using Sonar’s “Bounce to Clips” function. Most of the time it works fine, but sometimes the Bounce to Clips will create a blank clip – a clip with a horizontal line going through the middle. Strangely, I can still here the clip's audio on playback, I just can’t see a waveform.
Has anyone else experienced this?

No, but try dragging the edge of the clip you see; the audio you hear might be hidden behind it.

Mute? Solo?

Possibly a corrupt picture file
 
To recompute, right-click on the clip, click "Associated Audio Files..." at the bottom of the context menu then click "Recompute Picture(s)" on the Associated Audio Files dialog.
 
If the picture cache has never been cleared, it might be a good time to it. The location of the Picture Folder may be found in Preferences > File > Audio Data. In Windows, go to the folder and delete all of its content.
 
To render Region FX, I use the "Render Region FX" option on the Region FX menu under the FX in use.

Thank you Greg and Gswitz and Scook for your responses.
 
I followed your instructions, Scook, going to "Associated Audio Files" and then "Recompute Pictures" and I'm pleased to announce that it worked.
 
So, Scook, not only have you restored my waveform, but you have restored my faith in this forum. I think I was a bit unlucky with a couple of recent posts about issues that didn't get much response, and didn't get resolved. So thank you again.