The protected low
In an uptrend, the higher low that produced the latest BOS is the protected low. Buyers must hold it. When price closes below it, the character of the market has changed: it is no longer making the pattern that defined the trend.
Warning, not reversal
One CHoCH does not prove a reversal. Many trends pause, break a low, and resume. Treat it as the moment to stop looking for buys and start watching whether price can make a lower high and then a new lower low.