Well when you can't open a web site this could be for 3 reasons.
1- You ISP's DNS server is unable to resolve the address of Yahoo.com, and so forth this DNS server does not provide your computer with the correct address of yahoo.com website.
2- Your DNS Server is fine but there is a problem in the connection between your computer and yahoo.com.
3- Someone blocked this website on purpose whether from your computer or from Your ISP, which is something I doubt.
I am gonna let do a small test.
Open Start/Run then type: "cmd" then hit enter
type: "tracert yahoo.com" then hit enter
this will eliminate the first 2 possibilities.
If the tracert command gives you "Trace Complete" in the end, This means that there is nothing wrong with your DNS or your connection and the problem now is with your computer itself.
I hope this will help you