When a website goes down, working through checks in the right order saves critical time. This checklist moves from the fastest, most common causes to deeper investigation.
Step 1 — Confirm It's Actually Down (Not Just for You)
Use a third-party "down for everyone or just me" checking tool, or ask someone on a different network to test. If it's only down for you, the problem may be local DNS caching or your own ISP — not the server.
Step 2 — Can You Still SSH/RDP into the Server?
ssh deploy@YOUR_SERVER_IPIf you can't connect at all, see VPS Unreachable/Can't Connect via SSH: Troubleshooting Guide — the issue is likely at the network or server level, not the application.
Step 3 — Check If the Web Server Is Running
sudo systemctl status nginx
sudo systemctl status apache2If stopped, start it and check why it stopped:
sudo systemctl start nginx
sudo journalctl -u nginx -n 50Step 4 — Check Disk Space
df -hA full disk is one of the most common causes of a site suddenly going down — databases and web servers often fail silently when they can't write. See How to Fix "No Space Left on Device" Errors.
Step 5 — Check System Resources
free -h
uptime
topHigh load or exhausted RAM can make a server unresponsive without technically being "down."
Step 6 — Check the Application/Backend
docker ps
sudo systemctl status your-app-serviceIf using a reverse proxy (Nginx in front of an app), confirm the backend process is actually running:
curl http://127.0.0.1:3000Step 7 — Check the Database
sudo systemctl status mysql
sudo systemctl status postgresqlA crashed or unreachable database is a very common root cause of a site that "was working fine."
Step 8 — Check the Error Logs
sudo tail -100 /var/log/nginx/error.log
sudo tail -100 /var/log/apache2/error.logThis almost always identifies the exact failure — the fastest path to a real answer, not guesswork.
Step 9 — Check DNS
dig +short yourdomain.comConfirm it resolves to the correct IP address; DNS misconfiguration or an expired domain can look identical to a "server down" scenario.
Step 10 — Check SSL Certificate Expiry
sudo certbot certificatesAn expired certificate can cause browsers to block the site entirely, appearing as "down" to visitors even though the server is fine.
Step 11 — Check the Firewall
sudo ufw statusConfirm ports 80/443 haven't been accidentally blocked by a recent change.
Quick Reference Diagnostic Table
| Symptom | Most Likely Cause |
|---|---|
| "Connection refused" | Web server not running, or firewall blocking |
| "502 Bad Gateway" | Backend app crashed or unreachable |
| Site loads but very slow | High CPU/RAM usage, or database bottleneck |
| SSL/certificate warning | Expired or misconfigured certificate |
| "This site can't be reached" (DNS error) | DNS misconfiguration or domain expiry |
After the Incident: Prevent It Next Time
- Set up basic uptime monitoring so you're alerted before customers report it
- Enable automatic security updates — see How to Enable Automatic Security Updates on Ubuntu & Debian
- Monitor disk space proactively — see How to Check and Manage Disk Usage on a Linux VPS
Related Articles
- VPS Unreachable/Can't Connect via SSH: Troubleshooting Guide
- How to Fix Common Nginx Errors (502/504/403)
- How to Diagnose a Slow VPS: Complete Performance Checklist
