Audi A5 ignition wont turn on?

[Jono202]

Could anyone shed any light on this please really scratching my head with it. One morning it fired up and ran like a dream I drove home parked up came back to it an hour later and now the ignition wont turn on with the push in key? Really strange has anyone had any issues like this? The key fob works fine unlocks all doors as it should. Even tried the spare key and that's exactly the same! Any help is more then welcome it's a 2.0 diesel sport back Audi A5 thanks!