Apache2 Debian default page showings instead of site

The issue you're experiencing with the Apache2 Debian default page showing instead of your client's website on Amazon Lightsail is a common problem, especially after instance reboots or when creating new instances from snapshots. This typically occurs due to conflicts between different Apache services running on the instance.

Lightsail instances, particularly those using Bitnami stacks, come with pre-configured web services like Apache or NGINX. When you install or update packages, it's possible that a new Apache2 service gets installed as a dependency. This new Apache2 service can start automatically on port 80 after an instance reboot or stop/start, preventing the Bitnami-provided Apache from starting and serving your website.

To resolve this issue, try the following steps:

1. Stop and kill the Apache2 service that's causing the conflict.
2. Start the Bitnami Apache service.
3. Disable the conflicting Apache2 service from starting on boot.

If these steps don't resolve the issue, you can try restarting the Bitnami scripts directly by running the following command:

sudo /opt/bitnami/ctlscript.sh restart

This command may help reset the IP address correctly and restart the appropriate services.

If you're still experiencing issues after trying these solutions, it's possible that there might be other configuration problems or conflicts within your instance. In that case, you may need to review your instance's configuration files or seek further assistance from AWS support.

Remember that when creating new instances from snapshots, some settings might not automatically update, such as the site URL in WordPress settings. You may need to manually update these settings to reflect the new instance's IP address or domain name.

Sources
Turn off Apache default page on Lightsail Bitnami | AWS re:Post
Can't login Wordpress hosted on new Lightsail instance created from snapshot | AWS re:Post