I encountered this last night as well after a Windows update.  Not sure how to fix it, but what I did was download a new version of the miner and copied over my start.bat file.  I then stripped out all of my mod'd parameters that I use in the miner so it was just my cards and nothing else starting up and rebooted the machine a couple of times and "poof", started working.  The "poof" was the magic part because I still don't know why I wouldn't work in the first place.  Once it started up I went back in and applied all my modifications.