Exchanges halt a stock for pending material news, for extreme volatility (a Limit Up / Limit Down halt, typically 5 minutes), or by regulatory order. During a halt you cannot trade, and when it reopens the price may be far from where it stopped.
Halts are one reason stop-loss orders do not guarantee an exit price. If a stock halts at $20 and reopens at $14, your stop at $19 fills at $14.
Example: a volatile small cap trips a volatility halt at 10:32 after moving 10% in five minutes. It reopens at 10:37 with a new opening auction price.
Related: circuit-breaker, gap, stop-loss, slippage