It's a common failure with so many causes. Unless it happens daily we reboot and carry on. If it is happening daily the DNS is at your ISP so it could be overloaded (most common.) Try using the Google DNS (google "How to use the Google DNS?")
I get on the internet.Then I use my browser to look up a site. At the bottom of my screen a message appears . It says "resolving host" Then it says the site is unavailable. No matter what site I try to get to it does the same thing. I put on Windows 7 network troubleshooter. It comes back with a message that says "DNS server not responding." I then restart the computer and try it again and everything is OK.
Why is this happening??