Prius C wouldn't start after empty fuel tank

I have just bought a second hand 2012 Prius C iTech from a Toyota Dealer and hand problems starting the car after I ran out of fuel. I am still getting used to the fuel gauge and how far I can go when the light starts flashing so won't make that mistake again.

Anyway, reading through these forums has indicated that running out of fuel is a pretty bad thing to do in a Prius. I was driving along a back street and noticed that the petrol engine was no longer cutting in and that I was on battery only. I immediately turned around and headed for the nearest fuel station running on battery only. I managed to get within about 700 metres of the station before the battery ran so low that it would run any more :-( I got my wife to bring some fuel (about 5-6 litres) and filled the tank but it wouldn't start. I was facing upwards on a slight slope. When I tried pressing the start button with my foot on the brake as usual it would go into start mode but when I tried to shift into D (Drive) I kept getting the message saying that I had to put the gear back into P (Park) to start. I tried the emergency start procedure on Page 413 of my manual but that did not work. It appeared that both the 12V and traction batteries still had some charge in them as all of the panel displays worked properly and the traction battery showed 1-2 bars of charge. I called my emergency roadside assistance company to come and help but while I was waiting I tried again and the engine started.

Can someone give me some insight into why I could not get it to start after putting a reasonable amount of fuel in? Is there something I can do if this ever happens in the future? I will be extra careful to never run out of fuel again but you never know what may happen (we recently had a multi-day power outage in our area and local service stations could not pump fuel so this could happen through no fault of my own.)

Thanks.