Driver issue mostly. You need to reinstall the scanner driver. Or you can have a quick scanning check as suggested in http://kb.dynamsoft.com/questions/881/%5BTroubleshooting%5D+Unable+to+open+twain+source..
I'm running a Win 7 platform and have tried to install a CanoScan 4200F flatbed scanner.
I've downloaded the canoscan driver for Win 7 and am able to get the "toolbox" on my screen. However, when trying to scan a piece and send it as an e-mail, I get the following,"Unable to open the twain source" and a subsequent message appears,"Check connection and restart tool box" .
For some reason, I have to unplug the power source and replug to see light under the scanner cover. After a short period of time, the light goes out. I suspect the scanner (4 yrs old or so) has failed.
Is there anybody out there than might have a clue as to what's going on?
Looking forward to a reply.