NATS or something else?

I have a 2008 S6 TB48 that was running fine until trying to start it today.

Starter is cranking but engine doesn't fire. When key is turned to ignition, there's a general whine from the front end of the vehicle, but can't seem to isolate the sound of the fuel pump priming (though I've never actually listened to hear what it sounds like in this vehicle) and fuel pump relay doesn't click at all. Switch to start and the fuel pump relay clicks, then clicks again when key is released back to ignition position.

I did have the P1612 code (NATS) come up twice about a week ago. But it seemed to go away by itself (though now I'm second guessing myself if I somehow accidentally cleared the code using the Torque app without fixing the issue). NATS light is flashing until key is turned to ignition, then it turns off.

Other thought is possibly the crank sensor?

Any ideas?

Not sure how to fix it but P1612 code is the anti theft system issue. The systems immobilizer is not communicating with the chip on your key. Hence the car cranking and not starting.
Is the light on the top of the dash flashing irregularly? Or constantly on? I think this is a tell tale sign too.

No, only flashes normally when no key. When key is inserted and turned to ignition the light goes off.

Have you got another key to try?

Yep, tried them both with no success.