Hi, I had the exact same issue with my Lexus RX400h a few weeks ago. All power was suddenly lost (kids in car and just off M25 so lucky escape). It was kept in Lexus Twickenham for over a week before they sugested that it needed an Engine ECU at a cost of over £3000. That was after replacing the front speed sensors to find that this dodn't fix the issue. I wrote to UK Customer Services Director and got a standard letter back saying "tough luck".
This is a company with a history of failres and recalls and some suggestion that they've hidden known safety issues in the past. I have started a Facebook page to alow people to share their experiences of this nature. https://www.facebook.com/pages/Lexussafetydangers/. I suspect that we are not alone and that more people will come forward with tales that might undermine some of the reputation they've built-up for safety and customer service excellence.