What steps will you take to diagnose the cause of application unavailability after receiving a message about its unavailability?

To diagnose application unavailability, I would sequentially perform the following steps:

1. Check the server and service status where the application runs (e.g., using systemctl, docker ps, kubectl).
2. Check application logs and system logs for errors or warnings.
3. Check network connectivity to the server (ping, traceroute) and accessibility of necessary ports (telnet, nc).
4. Check firewall configuration and security rules (firewall, security groups).
5. Check the status of databases and other application dependencies.
6. If a load balancer is used, check its status and routing.
7. If necessary, reproduce the problem locally or in a test environment.

This systematic approach helps quickly localize and eliminate the cause of unavailability.