my ekg app disappeared on my apple watch 5

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 

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.

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

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

2 questions:

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

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.