My apple watch used to ring when the iphone alarm went off but not long ago it stopped ringing

Be aware that your watch will not get alerts if your phone is unlocked. If you are using the phone it is reasonable to assume you have seen the notification. Notifications on your Apple Watch - Apple Support

If that isn’t the solution you need and restart has failed to fix the problem the next escalation would be to unpair the watch, then re-pair and setup again. However this is hard work, so I suggest you make a final check on your watch speaker and settings. Does the watch still have good audible alerts for other events like messages and incoming phone calls?

Hi

Go to watch app on iPhone check sound settings sounds& haptics

Cheers

Brian 🇬-1F1E7;