Website down and don't know why

Are you running this on EC2 or Lightsail? And when you rebooted the site was that just a restart, or an actual stop & then start? If it was the latter then the public IP address & its DNS name will almost certainly have changed.

Which address(es) does your security group (instance firewall for Lightsail) allow inbound access on ports 22, 80 & 443? If it was originally setup to be limited to your IP then has that changed - depending on your ISP your home IP address can change periodically, and obviously your IP is different at home/at work/on VPN/on cellular. Try opening it up to 0.0.0.0/0 just while you're troubleshooting, to see if that helps.

If you can connect to it alright but the site if still going offline then look at resource usage. There will be a limited amount of information in AWS Console, to get more details information look at installing CloudWatch Agent on EC2 or Lightsail.

Hello,

It sounds like you may have load exceeding the sustainable capacity of your instance. Lightsail instances do allow periods of burst capacity, but if the instance remains in that higher load zone for too long it will reduce performance, potentially causing the issues you describe. Please see if this documentation helps you confirm or rule out bursting as your issue.

Bursting documentation: https://lightsail.aws.amazon.com/ls/docs/en_us/articles/amazon-lightsail-viewing-instance-burst-capacity

If bursting is the cause, your best bet is likely increasing the instance size. This guidance explains the process of creating a snapshot of your existing instance and then using the snapshot to create a larger one. https://lightsail.aws.amazon.com/ls/docs/en_us/articles/how-to-create-larger-instance-from-snapshot-using-console