Hey there norman258,
It sounds like you are unable to use your Messages application becuase it is asling you for your password, but it is not saving or recognizing that it is correct. I would use these troubleshooting steps to help resolve the issue:
To resolve issues with sending and receiving iMessages, follow these steps
- Check iMessage system status for current service issues.
- Go to Settings > Messages > Send & Receive and make sure that you registered iMessage with your phone number or Apple ID and that you selected iMessage for use. If the phone number or Apple ID isn't available for use, troubleshoot iMessage registration.
- Open Safari and navigate to www.apple.com to verify data connectivity. If a data connection isn't available, troubleshoot cellular data or a Wi-Fi connection.
- iMessage over cellular data might not be available while you're on a call. Only 3G and faster GSM networks support simultaneous data and voice calls. Learn which network your phone supports. If your network doesn't support simultaneous data and voice calls, go to Settings > Wi-Fi and turn Wi-Fi on to use iMessage while you're on a call.
- Restart your device.
- Tap Settings > General > Reset > Reset Network Settings on your iPhone.
Thank you for using Apple Support Communities.
All the best,