Down for everyone or just you?
If our check succeeds but the site does not load for you, the problem is likely between you and the site: DNS cache, a VPN, a firewall or your ISP. If our check fails too, the site is down for everyone.
Common reasons a site is down
Expired SSL certificates, expired domains, a full disk, a crashed database, a bad deploy, DNS changes that have not propagated, or a DDoS. Monitoring tells you which one, and when it started.