I leave the slideshow running as I leave for one hour of Sunday School, but when I return to the sanctuary for the main service, the SongShow Plus has crashed and the desktop is being projected.  I restart it. No problems with it during the services thereafter.  Just when unattended.

It is most likely that something occurred on the machine while it was unattended (sleep/update/display change etc) that caused the issue.  

If you can (or try) replicate the behavior, please pay close attention to what happens on the machine while unattended that does not happen when it is being actively used.

FYI - if you have a need to run slide shows unattended for extended periods, I suggest exploring our SongShow Signage solutions.