How cold is it where you ride? This can happen if grease in the hub is too thick for the cold weather. The pawls inside the hub are stuck and don't engage so the cassette spins freely every which way. Temp fix is to bring the bike inside to let it warm, but if this is a recurring problem, you should apply thinner lubricant. See this Icebike article for details.

I'm not sure that can cause the other problem you described though, where your bike essentially becomes a multispeed fixie. I've had that happen to me once, but that was because one of the bungee chords caught rear wheel and jammed between cassette and hub. Pretty much what Jeff Wills described above.

--J