Hi – Major Indices are quoted 24hr/day until Friday close with IG. So that is why you were stopped out unfortunately. Even when the exchanges themselves are temporarily closed to switch over to the night sessions, IG will continue to make a market during this period.
In your case it was pure bad luck as the news of the resignation of Gary Cohn was announced just after market close and the futures reacted violently to the downside especially as liquidity is low in overnight trading. In hindsight your stop was at a decent level as it traded as low as 24,411 in the following couple of hours 🙂