This EXACT same thing just happened to my little duck Vinny about three weeks ago. He was in a play pen, and all of a sudden fell over onto his stomach with his head all the way bakwards, stiff as a bored. After about 10 seconds he Rebooted and went back to normal. He's been fine ever since... probably a seizure. we thought maybe vinny slipped on the tiles in his pen and hit his head, but we never knew for sure.