I thought the shutdown was to try to prevent a crash by shutting the whole thing down. In other words - it was intentional.
The will intentionally shut down the trading floor but not the trading systems. You will still be able to execute trades minues a 1-2 day disruption to switch to emergency back up systems.
Believe it or not there are still trading pits where people stand inches away from each other all day. If one of them gets sick the whole crowd is getting it. They may shut down the floor portion of the exchange but the trading systems will keep running.
And no, other than circuit breakers or temporary stops during the day, they will not shut down the exchange to prevent a sell off.