I had an unexpected shutdown

Without a whole lot of additional information (about what you were doing. what you would expect to see as a resolution, etc.), the answer is:

Power up your Mac.

I have just encountered this problem for the first time..went to make a coffee and when I came back the screen was even blacker than mycoffee I had to hold down the on/off button and force a restart found this thread https://discussions.apple.com/thread/5552936http:// illuminating (if you will pardon the pun) but also very worrying as I will be using my Air for public presentations.

I have reset my energy saving preferences from sustainable to Nuclear powered hoping that if the issue is related to sleep I can atleast get through my presentation before my Air needs a rest.

Brand new Macbook air 11 inch, customised, 512 Gb, 8 MB ram, Fastest processor available, Mavericks update.Already 2 enexpected shutdowns, when display goes to sleep and not waking to any button for a while. After mutiple attempts, wakes up and works well.

This article may shed some light (pardon the pun):

http://www.macrumors.com/2014/02/18/sleep-wake-crash-macbook-air/

Thanks for the link...good article and seems to explain problem.

Thanks for the Info mate!