I've had the exact same problem. I believe it occurred to me when hitting the start button too quickly after inserting the key (insufficiency deep or giving insufficient time for the electronics to recognize the key). The key then doesn't pop out because the electronics are "hung". Anyway, I solved the problem by disconnecting the battery for a few minutes.
I made the mistake by trying to disconnect the positive. That took a long time, dropped a nut behind the battery, ended up having to take out the battery, etc.. It was then when I noticed that disconnecting the minus would have been much easier...
Nowadays, I wait one second before hitting the start button. Hope this helps.