Work through the most common causes in order.

1. Is it just you? Test with a "down for everyone or just me" tool, another device, or mobile data. If only you can't reach it, clear your cache (article 199) or check your network.
2. Account suspended? Check for an overdue invoice (article 198).
3. Domain expired? Confirm your domain is still registered and renewed (article 49).
4. DNS issue? If you changed nameservers or records recently, allow time to propagate (article 137).
5. Server error? Look for a 500/403 message (articles 193, 196) or a known outage (article 192).
6. SSL warning? See article 142.

Tip: Identifying which of these it is points you straight to the fix. If you're stuck, open a ticket with the exact symptoms (article 191).