Wordpress website (bitnami package) running on Lightsail instance stops displaying after 2-3 days

This is consistent with the host running out of a system resource and becoming unresponsive. My first guess would be memory, maybe a piece of software you are running has a memory leak, or just the workload on the server needs more and more memory as time goes on, and after 2 or 3 days there's none left (strictly speaking what happens is the memory manager comes into play, and will try to swap processes out of main memory and onto disk, which in itself is CPU-intensive. As free memory gets less and less the memory manager will spend more and more CPU trying to free up pages of memory, leaving fewer and fewer CPU cycles for anything else, such as listening on ports 22, 80 & 443. Which is why the instance appears to be inaccessible)

First thing I would do is look at setting up CloudWatch Agent https://repost.aws/knowledge-center/lightsail-monitor-with-cloudwatch to keep track of memory and see if that is causing it.

Another possibility is that your website is being attacked by a DDoS (Distributed Denial of Service) attack. A DDoS attack is an attempt to overwhelm a website with traffic in order to make it unavailable. If you are under a DDoS attack, you may see errors in your Bitnami diagnostic tool, such as "ports 22, 80, and 443 are not accessible remotely." To check if you are under a DDoS attack, you can use a DDoS detection service. If you are under a DDoS attack, you will need to take steps to mitigate the attack, such as using a DDoS mitigation service. It is also possible that there is a problem with your Bitnami installation. If you have recently made any changes to your Bitnami installation, such as installing a new plugin or theme, it is possible that these changes have caused a problem. To troubleshoot this issue, you can try rolling back your changes or reinstalling Bitnami. Check your Lightsail instance's logs for any errors. Use a network monitoring tool to check for packet loss or other network issues. Try connecting to your website from a different location. Try accessing your website using a different browser.