Q: In my Mac, most of the sites are not working

Please download and install EtreCheck from http://etrecheck.com/

Run it and post the report here.

What OSX version?

If earlier than Mavericks likely cause is that your browser has security flaws and thus sites do not allow you to see those sites.

Try this link.

Etrecheck – System Information

Safari - Unsupported third-party add-ons may cause Safari to unexpectedly quit or have performance issues

Safari doesn't load a page or webpage items are missing

Safari/other browsers – Website not loading