Nothing at all.
THE CHECKS, IN ORDER
Is the domain registered and not expired? A WHOIS lookup shows the expiry date. Are nameservers set? WHOIS shows them. Do those nameservers respond? Does the zone contain an A record?
THE MOST COMMON CAUSE
The domain expired. The site and mail stop and the data is untouched.
Open a ticket immediately; recovery becomes more expensive with time and eventually impossible.
THE SECOND MOST COMMON
Nameservers not set, or set to a provider that no longer holds a zone for the domain.
A newly registered domain may have the registrar's defaults, which point at nothing useful.
IF NAMESERVERS ARE CORRECT
Check the zone at that provider contains an A record for the domain.
A zone with no A record resolves to nothing.
IF EVERYTHING LOOKS CORRECT
You may be waiting for propagation after a recent change.
Query the authoritative nameservers directly. If they answer correctly, the change is made and caches are catching up.
IF ONLY YOU CANNOT REACH IT
Try mobile data. A firewall block produces a different symptom entirely.
