Something unforeseen is something that could not be predicted and was not expected. It's a surprise.
If something was foreseen, you saw it coming and it wasn't a surprise. Something unforeseen is the opposite: no one saw it coming. Accidents are usually unforeseen events: no one expects to get in a car or bike accident on a given day. Winning the lottery, since it's so unlikely, would be an unforeseen event. If something was unanticipated or out of the blue, it was unforeseen. Unforeseen events can be good or bad, but they’re all surprises.