ECG does not work on my watch 10

Any error messages given?

The easiest fix would be a force restart of your watch.

To force restart your watch, press and hold the Digital Crown and the Side button together for about 10sec, until the Apple logo comes back again.

If this does not work, and to rule out a software issue, erase the watch and pair it again, setting it up as a new device. You will only lose your created Watch faces and may have to activate the cards in your Wallet app again.

Then test the ECG again.

Unpair and erase your Apple Watch - Apple Support

Depending on what “doesn’t work” actually means (as in what happens when you try it), simply positioning the watch a little bit away from your wrist may help.

Can another person use the watch and start an ECG?

Did you also set it up as a new device, or just tried the forced restart?