Hello everyone, a newbie here based in the UK using Cubase Elements 8. I recently experienced this disappearing wavelength image problem too, and it was doing my head in. After finishing recording some vocal tracks and ‘playing around’ with them, several wavelength images disappeared all of a sudden, showing just a thin line. The audio was there and sounded fine, and the correct image appeared when zooming in or opening up the event. Still it bothered me, so i tried the various suggested fixes like deleting peak files & folders, but to no avail. Then by chance and out of desperation, i tried one more thing and it worked; now all is fixed and my project looks perfect again 
And the fix was - simply add some gain (an infintessimal amount will do it) to the offending event (Audio menu - Process - Gain) and hey presto, the proper wavelength image perfectly reappears, and it stays there You can even undo the add gain if you so wish, and the reconstructed image stays.
Hope this helps some of you reclaim your sanity as it did me 