Same thing happened to us, in case anyone is interested in finding out what worked for us.
Ours was the battery - completely dead, but because it's a hybrid the electronics all worked.
There was no indication that the problem was starting, no mistakes made with driving or the last time the engine was turned off. When the ignition was tried the next time, all the warning lights lit up and relays clicked away like crazy. If a similar thing happens to you then jump start and drive to somewhere you can replace the battery as soon as possible.