Go to device manager ,choose your canon device there and right click on your device.Choose, update driver,it will connect to internet to download the correct driver for your device.
Navigate to the Twain driver installation directory (typically c:\windows\twain32\). Delete the CNQLxx directory and reinstall the driver from the original disks or from the Canon website.
This scanner is sometimes a bit temperamental. Try unplugging it from your computer and restarting and then hook it back up to your system. This should clean out anything giving it a problem.