Put new plugs, leads and a coil pack on there and see what happens. When my wife's fiesta went down with similar symptoms, the ECU could also not be read. I had a coil pack in the garage so put it on knowing it is a common fault. Problem solved.

When the plugs wear, they require more amps to make a spark, so the coil pack is working harder which makes it fail. This can blow the ecu at the same time, but it is very rare. I don't know why the codes couldn't be read on our fiesta, maybe the ecu went into shut down or something to protect itself, who knows. But a new coil pack, plugs and leads sorted it. Worth a try!