It would help to know what model scanner you're using, which version of PS, and which software you're using to interface your scanner with PS. Are you using the scanner manufacturer's own drivers or specialized software like Silverfast?
Try unplugging, then replugging in the cable to your scanner. If that doesn't help, check taskmon and if your TWAIN driver software is running, turn it off. Reboot, then plug in the usb again and turn the scanner on. If it wants to add new hardware, hit cancel. Check PS to see if it loads properly this time. If it still gives you that error message, completely remove your scanner software using Add/Remove and re-install it. Make sure you have the latest version of your scanner software. Good luck.