Same thing that happens if a brake line pops, steering wheel locks up, or the throttle gets jammed open. There are ways of designing redundancies into those systems. Multiple communication channels, reserve sources of power. A lot of new model vehicles already have this stuff, drive by wire for example has pretty much been the standard for twenty years now. Typically, the entire system doesn't go down because they're designed for that not to happen.
But I'll take my mechanical overrides.