ECG icon disappeared from my Apple Watch

Did you somehow delete the ECG app from your Apple Watch?

You can download deleted apps onto your Apple Watch again, using the AppStore on your Apple Watch.

Search for ECG app, you might have to scroll down to the bottom of the page to find the Apple ECG app.

Have a look at this article:

Get apps on Apple Watch - Apple Support

My wife did the same thing while we were travelling. I had to login to a local WiFi then go to the app store on her watch. I searched for ECG, downloaded it, and it runs without any problem again.