To resolve the issue of the Planet Fitness app not opening on your phone, follow these detailed steps:

1. Check for Software Updates: Ensure that your phone's operating system is up to date. Go to Settings > Software Update and check for any available updates.

2. Update the Planet Fitness App: Make sure you have the latest version of the Planet Fitness app. Open the Google Play Store, search for "Planet Fitness" and see if there’s an "Update" button available. If so, click it to update the app.

3. Clear App Cache and Data:
- Go to Settings > Apps > Planet Fitness.
- Tap on Storage.
- First, select "Clear Cache" and then "Clear Data". This will reset the app and may resolve loading issues.

4. Force Stop the App:
- Navigate to Settings > Apps > Planet Fitness.
- Tap on "Force Stop". After that, try opening the app again.

5. Restart Your Device: Sometimes, a simple restart can do wonders. Turn off your phone completely and then turn it back on.

6. Reinstall the App: If the above steps didn’t work, try uninstalling and then reinstalling the app:
- Go to Settings > Apps > Planet Fitness.
- Tap on "Uninstall".
- Open the Google Play Store and reinstall the app.

7. Check Your Internet Connection: Ensure your phone is connected to a reliable internet source, whether through Wi-Fi or cellular data.

8. Check for Known Issues: Sometimes the app might have widespread issues. You can check forums or the Planet Fitness FAQ page for any announcements regarding app problems - learn more.

9. Contact Support if Necessary: If none of the above steps work, consider reaching out to Planet Fitness customer support for assistance or checking their official social media channels for updates regarding the app - learn more.

By following these steps, you should be able to resolve the issue with the Planet Fitness app not opening on your device.