What is your OS? Do you have the full featured software installed and set up (TWAIN driver plus the Toolbox including ScanGear CS )? Do you mean the software scan button or the hardware (on the scanner) button?
If you are trying to launch a scan from the scanner, then scan button will only work if the computer has the full software and is running the monitoring program. You also need the default destinations in the software. These are the standard resolution for the scan and the folder to save the resulting file among other things. The full software is available for supported OS versions here:
http://usa.canon.com/cusa/support/consumer/scanners/canoscan_series/canoscan_lide25?selectedName=DriversAndSoftware#DriversAndSoftware . Test scanning from the computer software as well.
If the problem is at the software on your computer, try a new data cable. If you are using a USB hub, try a direct connection. Scanners require bi-directional communication with the computer and will fail if there is any damage to the USB port, driver or cable. If you can, try a different USB port. If problems persist, disconnect the data cable between the computer and the scanner. Uninstall the Canon software and the driver. Then reinstall and try scanning again. (For Windows OS versions, check the Device Manager to make sure that your USB driver is working. Click Start > Control Panel > Hardware (title varies with version) > Device Manager. If you see a triangle with a red X or yellow exclamation point, try updating the driver.)
I hope this helps.
Cindy Wells
