The only thing on my machine that's changed is the Avid version, but when I try to export to Quicktime reference, I get files that are entirely black, or I get an error that I need additional software from Quicktim to continue.  So, if anyone has any ideas why this would suddenly happen or how to fix it, I really need your help.  Because my Avid is less than worthless to me until I can export my files. 

Also, if my method of exporting is DOA, I would love any alternative that will succesfully get media out of Media Composer. 

EDIT: following the advice in another post, I found the native mp4 export now in Avid, which is what I'll end up using.  

QuickTime Reference exports are MOV files that link to the media inside the Avid MediaFiles folder. In order for the video to be playable it needs to be a codec that QuickTime understands and be uniform through your sequence. What codec is the media in your timeline? Is your Avid MediaFiles folder local on a drive or remote on shared storage? Did the drive letter where the Avid MediaFiles folder is located change? If it was on g: drive yesterday and that drive letter is h: today then the link will break.