my ekg app disappeared on my apple watch 5

If you still don’t see the app on your Apple Watch, you can search the App Store on your Apple Watch for ECG and download it.

Try Force restart iPhone - Apple Support

On Watch try un-pair then re-pair with the iPhone: Set up and pair Apple Watch with iPhone - Apple Support

Update your watchOS to 6.2.1 if you haven't yet.

Do you have the ECG app now?

If you don't, did you try downloading directly from the Watch App Store?

FYI" There's no reset, you can only delete the health data.

Thanks for asking about the new ECG feature.

Check here for availability in your region: ECG

ECG requires the current non-beta version of iOS and watchOS. For additional help with the ECG feature on your Apple Watch, take a look at these articles.

Heart rate notifications on your Apple Watch - Apple Support

Taking an ECG with the ECG app on Apple Watch Series 4 or later - Apple Support

If you have any other questions please reach out to our team here: https://apple.co/HeartFeatures. If you believe that you are experiencing a medical emergency, please dial your local emergency services.

Take care 

Which iPhone model & iOS version do you have?

ECG see here: Taking an ECG with the ECG app on Apple Watch Series 4 or later - Apple Support

2 questions:

1. In which country did you buy the Watch?
2. Where are you using it?