Your resolver did not answer
This message means the DNS server configured on your computer or router did not return a reply in time. It is often local: ISP resolver blip, router crash, or a bad custom DNS IP. It can also be that the domain’s own nameservers are down — DownSpotter will show a DNS failure in that case.
What to try
Reboot the router if several sites fail. Toggle Wi-Fi. If you set custom DNS, confirm the addresses. Avoid downloading “DNS repair” utilities; they are a common malware vector.