And was the duration field updated on the General tab after you started playback?
When the duration is unknown (= 0), the waveform cannot be displayed. You can verify that a duration is known by looking at the General tab.
When you start playback (on air or PFL), the duration will be updated automatically. This should be reflected on the General tab immediately. Then when you reopen the Cue Editor, the waveform should be back.
I think your issue might be relarted to this one (duration not read from files because options were unchecked):