Hi Liz, Sometimes it can help to unplug the scanner and then re-connect it to a different USB port on the PC. That way it may get re-installed and kick itself into action.
You also ought to have a look in the Device Manager (Right click My Computer - Select Hardware - Device Manager Have a look down the list of devices and you should see an entry for Imaging Devices. Check if this is present and if there is any exclamation mark or other error indication. If there is any error, double click on the entry and see what information you get out of the error text. Not always helpful, but a good place to start.
Also worth thinking about what software/hardware you may have recently installed on your PC that may be causing the scanner to stop working?? If there are any likely culprits, then try uninstalling these.