Don't know why you had a problem but, when whipping egg whites, cleanliness is paramount. You have to be certain your bowl and beaters/whisk are pristine and free of any oils, etc. Same for when you crack the eggs to separate them. NEVER let your hands/fingers/skin touch the egg whites because any oil will affect the whites when they are beaten.
Another thing that will give your egg whites volume is to bring your eggs up to room temp before breaking them. Beyond that, except for being interrupted, I don't know what happened to your mousse.
"As a girl I had zero interest in the stove." - Julia Child
This is real inspiration. Look what Julia became!