I’ve just bought a series 3 watch. It’s on earth week old. It’s been syncing the activity with my phone up until Friday when it stopped completely! I have tried turning on and off and now I have unpaired the phone. I can’t repair it with my phone

Hi

If your Apple Watch is currently unpaired according to your iPhone, but your Apple Watch still believes that it is paired, then use one of these methods to erase your Apple Watch before re-pairing it to your iPhone:

Erase your Apple Watch manually:

• On your Apple Watch: when viewing the time, go to the Home screen (via a single press of the Digital Crown) > Settings (gear icon) > General > Reset > Erase all Content and Settings > confirm when prompted.
• Unpair your Apple Watch and iPhone - Apple Support

Erase your Apple Watch via iCloud:

• On a computer, in a web browser, go to iCloud - Sign in and sign in with your Apple ID > Find My iPhone > Select All Devices > click on your Apple Watch > click on Erase Apple Watch > select Next until the device is erased.
• About Activation Lock on your Apple Watch - Apple Support

If necessary, erase your Apple Watch whilst charging it:

• Connect the Apple Watch to its charging cable, connected to power.
• Whilst keeping the watch on the charger and connected to power:
  • Press and hold the side button until you see the Power Off slider.
  • Press firmly on Power Off (do not try to slide it - just press down more firmly than a normal tap), release your finger, then tap on Erase all content and settings and confirm when prompted.
• If you forgot the passcode for your Apple Watch - Apple Support

With both devices unpaired, before re-pairing your Apple Watch with your iPhone, be sure to first check that your iPhone is updated to the latest version of iOS:

Update your iPhone, iPad, or iPod touch - Apple Support

Then follow the remaining instructions here, if necessary including contacting Apple Support for assistance via the link within the article:

Set up your Apple Watch - Apple Support