NXDOMAIN
The name does not exist in the zone that is authoritative. Causes: typo, record on www when users type the apex, deleted zone after a registrar lapse. Confirm NS, then whether the hostname is actually in that zone.
No records / empty answer
The name exists (or the parent says it does) but this type is missing. Asking CNAME on an apex that only has A will look empty. Asking A on a pure CNAME name may show the CNAME instead of an IP depending on the resolver. Try ALL in the lookup tool.
SERVFAIL
The resolver gave up: lame delegation, DNSSEC validation failure, unreachable authority, or a truncated answer that TCP did not save. Check NS and SOA. If DNSSEC is on, check signature expiry.
Split brain
Two hosts both think they are authoritative. The registrar points at one; you edited the other. NS lookup vs panel URL — they will not match.
Mail-specific
MX to a name with no A/AAAA. Two SPF records. DKIM selector mismatch. DMARC on the wrong host (apex instead of _dmarc). Use the mail tools rather than guessing from a generic TXT dump.
Local lies
Hosts file, stale OS cache, VPN DNS, ad-blocking resolvers rewriting names. If DNSFetch (server-side) and your laptop disagree, believe the server first, then flush the stub. Caching guide.
Walk-through: how to check records.