You cannot get your ScanSnap S510 to go online. You have a blue button with a white S in system tray but it has a red circle and a red line through it. You have turned power on and off, rebooted that computer but nothing helps.
You can try connecting that scanner to a different USB port. It's recommended to power off that computer first and then unplug that device from an existing USB port and plug it into a different USB port. Hopefully, when you power on that computer, Windows XP will find that device again and it will work properly.
You can go into Windows device manager by going to System Properties and then a Hardware tab and make sure there isn't any problems with that device driver for your scanner. If there is a yellow exclamation point then you will need to reload that driver. If there is a red x then you will need to re enable that device by right clicking and select enable.
Once in device manager and you see no problems with a device driver, you can try highlighting and deleting that scanner device by right clicking and selecting "delete". There is a "Scan for hardware changes" icon in a device manager window. If you click that icon Windows will scan and find that scanner again and will try to reinstall a device driver. You could also try to update that scanner driver from within device manager instead.
Trying to update that device driver or reinstall that driver might be a quickest way to fix your problem.