My Lenova Yoga 920, which I just purchased about a month ago, has tonight shut down thrice, and after shutting down has displayed a screen reading:
Default Boot Device Missing or Boot Failed
Insert Recover Media and Hit any key
Then Select 'Boot Manager' to choose a new Boot Device or to Boot Recovery
Sounds like the storage device has either become disconnected or failed. If the storage device is user accessible then open it up and make sure its seated properly, if its not (a lot portables aren't) then you will need to contact Lenovo for repairs/replacement.