Yesterday, I turned on my VR-500. It showed a low battery indication, so I turned it off and replaced the batteries. When I turned it back on again, the screen was blank, and none of the keys worked. It did show the "S" icon at the top, but that was it. I replaced the batteries with another set, but still got a blank screen. I bought this thing new two years ago, and I don't use it very often. What happened?