But as you had changed your domain name your visitors were not aware of it and they we will open your old domain, so you need to redirect them to new address or domain.
You can put a auto-redirect page on your root directory of old domain but this will not work if your visitor try to open sub-page or any post.
So in order to redirect all pages to your new one you had to open and edit your .htaccess file and delete every thing from it and add following line to it :-
Redirect 301 / http://www.YourNewDomain.Com/
Save it and then upload it back to your server, and now open any of your page like www.YourOldDomain.Com/about, then you will be redirected to www.YourNewDomain.Com/about.