Canoscan won't scan because the other program (?) is busy. I am not aware of any "other program" let alone it being busy.

First thing you'll want to do is reboot and try again. If you do that and you still get the same thing, then you'll need to see what other programs may be running, Ctrl+Alt+Del and then Task Manager (assuming you're in Windows) to see what other programs are running. Close other running programs. Yoiu may also need to identify any hardware or software that you may have recently installed (especially other scanning or TWAIN devices such as scanners, All-in-one devices, digital cameras, web cam, etc.). You may need to uninstall and reinstall the scanner software, though I wouldn't expect that to cause the scanner to be busy.