My Apple Watch and iPhone are not paired

What tells you it has lost pairing? A red phone icon is the usual way.

In iPhone My Watch tab, All Watches, tap on (i) if it says unpair then it is paired OK.

But you can unpair and pair again.

Unpair your Apple Watch 

First unpair your old watch to saso remove data and activation lock

 Unpair and erase your Apple Watch

Set up your Apple Watch

Then set up your new watch

Set up your Apple Watch – Apple Support (UK)

Red dot means you have a notification. Swipe down from it