If your watch becomes temporarily disconnected from your iPhone via Bluetooth and cannot connect to a known Wi-Fi network at the time, it will show the Disconnected icon on the watch face:
Such disconnections can be normal, occurring whenever the paired iPhone is unavailable (such as during iOS updates or when your iPhone is out of range).
It is not normally necessary to unpair and re-pair your devices after this happens. They should instead reconnect automatically a short while after being brought close together again. If they do not, keep your iPhone and Apple Watch close by each other and follow these steps until they do:
For the best performance from your watch, keep both Bluetooth and Wi-Fi permanently enabled on your iPhone.
If they still do not reconnect, unpair and re-pair your watch. If your watch is unpaired according to your iPhone, but not according to your watch, follow these steps to re-pair your watch:
More information:
If your Apple Watch isn't connected to your iPhone - Apple Support