Please follow these steps to fix the problem with your Apple Watch's connection:
Make sure the Workout Live app is open on both Apple Watch and iPhone. Hold your wrist up for a few seconds. Please note that when not in an active workout session, the iPhone disconnects from the Apple Watch when the screen goes dark.
Both your Apple Watch and iPhone should have Bluetooth and Wi-Fi turned on.
You should have your Apple Watch close to your phone.
Quit the Workout Live app on Apple Watch and reopen it (quickly press the digital crown twice and then swipe left).
Make sure you give the Workout Live app all the necessary permissions in iPhone's Settings > Health.
Check if your Apple Watch is connected to your iPhone. If your Apple Watch is not connected to your iPhone, the easiest way to try to connect again is to restart your Apple Watch.
If you are using any other apps on your Apple Watch that are live syncing to your iPhone or tracking an active Workout, make sure to quit them and then reopen the Workout Live app. Noted that the Workout Live app on iPhone doesn't work with the stock Apple Workout app. You need to open the Workout Live app on Apple Watch.