A couple of traders have asked why US Indices sometimes stop trading, then restart. This could be because of ‘circuit-breaker’ limits designed to stop irrational panic or exuberance from destabilising the markets.
At fixed level, the exchanges impose trading halts (or in some cases you can only buy, or only sell). The limits are percentage based and vary according to the time of day. Here’s a good example based on the S & P 500 https://www.cmegroup.com/trading/equity-index/faq-sp-500-price-limits.html .
During a trading halt, you won’t be able to place a trade on the index.