It’s an adverb and it means in a way that was not expected, anticipated or foreseen. So if something happens totally out of the blue, then it happens unexpectedly.
always happens at the worst time when you REALLY want to log in to your account..... cause OMG you need to see if anyone has commented on your "cool new pix."
damn... another sorry! an unexpected error has occurred! wtf.... i need to click this again, and again, and again!!! sorry my ass!!!