I'm not an expert, but I tried all the advice I found here and it didn't help, so I tried something I do when other apps act up.
- Make sure your phone and computer are on the same wifi. (found in the advice section)
- Action Center > All Settings > Apps
- Find Your Phone app > Advanced options.
- Click Reset, Click Repair
- Open Your Phone app and try again.
Yup, that's all it took to get the app working again. Hope this helps.