Here is how I would recommend responding to the customer's email:
Dear Pat,
Thank you for contacting us about the missing sleep data for your baby. I apologize for the inconvenience caused by the recent system outage. I understand how important it is to have accurate sleep tracking for your little one.
The outage unfortunately impacted users with an older version of our app. An update was released shortly after that resolved this issue. I'd recommend updating to the latest version of the app, which should prevent any data loss going forward.
Please let me know if you have any other questions! I'm happy to help make sure you get the full functionality from our sleep tracking. Thank you for your patience and understanding. Don't hesitate to reach out if you encounter any other problems.
Sincerely,
[Your name]
The key points are apologizing for the inconvenience, explaining the cause, recommending updating the app, and offering to help further. The tone is understanding, helpful and aims to reassure the customer. Let me know if you would like me to modify the response at all!