I’ve posted previously about problems that I’ve had with my MacBook Air 13” (2011 edition).
An additional one that crept up was its maddeningly random inability to wake from sleep. I typically take it to a client site everyday (or elsewhere when I’m travelling) and put it to sleep for the commute each way, and on too many numerous occasions, it failed to wake up, requiring a full power cycle, and then hope I remembered to save everything before it slept.
This post details a fix that, after a week or so, appears to have resolved the issue. You have to go into System Preferences and select a startup disk.
Why does this work? I don’t know, and neither does the original poster, but it appears to work, at least for some.