1. A cable plugged into the pc/if jack on the scanner. Remove the cable.
Or...
2. A physical hardware failure of some kind inside the scanner. Inspect the jack to be sure something isn't stuck in there. You may need to take it apart to inspect.
Scroll down a few pages... I had this same problem about a month ago. Fellow posters had really good advice, and I tried them all. Sadly, none worked for me, so I sold mine on eBay for about half of what it was worth. Good luck though!