For future reference, there is a difference between your devices being "unpaired" vs them becoming temporarily "disconnected".
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 by following the steps in my previous reply.
More information:
If your Apple Watch isn't connected to your iPhone - Apple Support