Hi
Try a force restart on both devices:
Force restart Apple Watch: press and hold both the side button and Digital Crown for at least 10 seconds, until you see the Apple logo.
Restart your Apple Watch - Apple Support
Force restart iPhone: press and hold both the Sleep/Wake and Home button for at least 10 seconds, until you see the Apple logo.
Restart your iPhone, iPad, or iPod touch - Apple Support
Failing that, unpair and re-pair your devices, restoring your Apple Watch from the iPhone backup when offered the option.
Unpair your Apple Watch and iPhone - Apple Support
Set up your Apple Watch - Apple Support
To clarify the terminology, in case you need further help: once "paired", an iPhone and Apple Watch remain paired until they are manually unpaired. However, they can frequently become "disconnected" and automatically reconnected (via Bluetooth and Wi-Fi).