Did everyone that own a Line 6 Spider III just die? When I bought the same amp, this forum was thriving. Moreso than it is now. You would log on and there would be pages of settings for particular songs/bands. Tech support was constant. A very thriving forum. I doubt all those people just forgot what their amps did, or went away? The error you show was the most common problem with the III and given the popularity of the amp, was talked about quite often. Here's the simple fix, or bad news... Turn the amp off. Hold down the Channel A button and turn it back on. This will reset the systems electronics/computer. If it doesn't work. Your amp is dead.
If you're somewhat electrically inclined, you can remove the cover plates and see if there's a loose connection on a ribbon cable. But... I don't remember many coming back from the black boxes, if the initial reset didn't work.