One particular project crashing

In X2 I was listening back to an old project originally created in X1 but never properly finished. It played back OK.  On listening back I thought the project could be revived and I could start by re-editing the vocals.  The vocal trax (6 of them in a folder) had previously been bounced to a sub-mix and then individually archived.   In order to try and re-edit them, I deleted the vocal submix stereo track then 'un-archived' each vocal track in the vocals folder.
 
However, after doing this the project won't play back and X2 is crashing every time at the same particular point in the project, a few bars in, so I have to close it down.    Also, the PC needs to restart before Sonar X2 will open again.  I can't see any particular event at the point where it crashes.
 
I have deleted all FX from the vocal trax FX bins.  There was an FX insert bus which I’ve also deleted.  
 
Other projects seem to be playing back OK.
 
Any ideas as to what's causing this or how to diagnose the problem?  Thanks.