When a DNS server is not set, it means that the DNS server is not responding to DNS requests. This can be caused by a variety of issues, but is most commonly caused by a broken or missing network connection. If a DNS server is not responding, clients will not be able to resolve domain names. When this happens, users may see errors or timeouts when trying to access websites or services.
Additionally, if a DNS server is not set, it can lead to problems with network security. Unresolved DNS issues can allow malicious actors to access network resources or harvest information from infected machines.