Sorry folks, looks like I'll be answering my own question. Seems I was wrong. . .the problem was in the computer. I connected a different scanner which came back with an error message saying my scan attempt failed because whatever scan program it required was already busy. That meant the trouble was inside the computer, not the ScanMaker 5900. With no way to see what the jammed program was, I rebooted the computer and everything is back to normal.