Canon mf 4100 scanner drivers

One day the scanner on the MF4100 was working. The next day, the scanner disappeared. Printer works, so I know it is talking to it as does the fax. Tried re-installing drivers from toolbox. No good. Tried scanner installation wizard, and add device, no go.
It must be something so simple I just don't see it. Perhaps it's a brain fart om my part. Wouldn't be the first time!