I have the original Apple Watch(I bought it as soon as it was released in 2015). I have now realised that it is not tracking any of my activity data, and that it displays 0 for all of the 3 categories, even after a day of running and exercise. How can I resolve this issue?
Unpair your watch from your phone and re-pair it. It will reset the watch.
Go into the Watch app on your phone, tap your watch at the top of the app screen, tap the "i" button, then tap Unpair Watch.
After this is complete, re-pair the watch to your phone and see if your issues are resolved.
If that doesn't work, you may need to call Apple support or take it in to the Genius Bar.