Site suddenly down. Customers leaving, ranking dropping.
1. Quick check
From another device/internet. isitdownrightnow.com.
2. Status pages
Hosting status, Cloudflare.
3. DNS
nslookup, dnschecker.org.
4. SSL expired
5. Server resources
cPanel — CPU, RAM, disk 100%?
6. Error log
PHP fatal, MySQL connection refused.