DNS Error
The majority of internet users get their DNS server addresses from their internet provider, which happens automatically and is likely to be the case if you haven't intentionally changed your DNS server. When the provider's server is overloaded or simply malfunctioning, this can lead to a DNS Error.
Unfortunately, both viruses and anti-virus programs can lead to DNS errors. When the anti-virus database is updated, there can be errors that lead the program to think your computer is infected when it actually isn't. This, in turn, can lead to "DNS server not responding" errors when trying to connect.
Minor errors with your modem or router can lead to DNS errors, too. These can be easily fixed by restarting the device although the fix might only work temporarily. It isn't likely that your router and/or modem will produce DNS errors regularly, but if it does, getting a replacement may be the only solution.