Well first time Ive seen this in all the years Ive been using a Cakewalk sequencer. Only on one project with 2 bass tracks, saved it yesterday but today I load it and the sound is all there but just straight lines instead of waveform displays. Any clues out there.
Thanks Roy

try bounce to clip

Thanks martins guit, That worked nicely, but why did all that happen, never done it before.
Thanks Roy

Do you ever clear the picture cache? This troubleshooting page discusses how to recompute picture files and clear the cache. The page references another section of the help to optimize picture cache redrawing that may be of interest.
 

maybe try to clean your picture-cache-folder

Thanks guys done all that to be sure, pictures showing fine now. Would like to have understood why this could happen but as long as we have a cure I guess it doesnt matter. Cheers all.

also set the pic. cache size to something very high like 1000MB

Go to Edit > Preferences > Audio - Configuration File and increase the PicCacheMB value. This variable specifies the maximum size of the picture cache. The default is 500MB.
 
 

Here's another quicker way to get your waveform showing properly:
 
Right click on the track and select "Associated audio files" at the bottom of the context menu.  When that opens a list of the audio files in the track, select them all and click "Recompute picture(s)" and the visual waveform will re-render and you can see it correctly.  (But I also agree that clearing the picture cache is helpful.)

Wow didnt realise I was still getting replies on this, thank you all didnt mean to ignore you. Ive now increased PicCacheMB to 1000 as sugested so hopefully end of problem.
Thanks Roy