I just ran into the same issue. Line 6 Central was working fine — it detected my Variax USB interface and my Variax 500 without any problems. Then it prompted me to do an update, which I went ahead with. The update failed, and after restarting as suggested, my Variax no longer produces any sound.
Now I can’t seem to get Line 6 Monkey to run in order to reflash it, so I’m a bit stuck on how to fix this. Any ideas?