Ralph, this happens to me daily. I have an HP ScanJet 5300 that my machine can't find. I found by shutting down my machine (restart), the scanner can be found and initialized. Occasionally even this won't work so I force my machine to quit by turning off the power while the machine is on (not a good practice I'm told) but on restarting, it finds the scanner. Emails and phone calls to HP have gone unanswered. As soon as I stop scanning for 20 minutes, the scanner light goes out and I have to resort to the above methods to get it going again.
George