Hello everyone,
I experienced this same problem on Saturday. I thought the router was unusable, but after unplugging it, removing the USB stick and all of the cat5 cables, and probably having it off for a few minutes... I then plugged it back in without anything attached to it except for power, and it booted up.
So you might want to give that a try if this happens to you. I didn't do a hard reset or anything, I just left it unplugged for a bit, and when I plugged it back in, I didn't have anything but power connected to it.
I hope that helps someone, as I'm still not sure what caused it - or even what fixed it.
-Vito