Hi mate, do you get a certain error code or message? Here are some suggested troubleshooting steps to take:
Check the app is up-to-date
Clear the app data/cache from your phone’s settings
Uninstall/reinstall the ChatGPT app.
Reboot the device
If all fails, you can backup/reset your phone but, hopefully one of the above helps.
If then you’re still unable to open the app, I’d suggest logging a bug with OpenAI.