How do I fix my computer that has a DNS error?

I have recently lost the ability to connect to a web-site that I need for work. Also i lost my wireless connection to the main computer in the house. It to cannot get to the web-site. All the rest of the internet works perfect as far as I can see. I am running a line directly to the modem now with the same problems. I has turned off modem etc.. and still have the problem. I have been able to connect to the web-site on computers outside so I aa lost as what the problem is?

I get the DNS error and have looked on forums and cannot find what the problem may be. iT sounds like something simple as far as just something need to be reset?

I run windows 7

Thanks for the help