A DNS server not responding can mean a few different things. The most common reason a DNS server might not be responding is because it is overloaded.
If the DNS server is being inundated with requests from clients, it might not be able to keep up and provide the requested information.
Another possible reason a DNS server might not be responding is if the server is down. If the DNS server is down, it cannot provide the requested information to clients.
In some cases, this might be because the server is physically down, or because the network connection to the server is down.
If a DNS server is not responding for other reasons, it might be because the server is configured to not respond in that situation. For example, a DNS server might be configured to not respond when it is down.