The first place I would look is with the DNS for your hosting. It should accommodate domains both with and without the www subdomain.
If your home page is working, but your post pages are not, you might want to make sure that permalinks are working. It could be that your .htaccess file has not been set up properly or your host does not allow the use of server overrides using the .htaccess file. If you cannot do URL rewrites, permalink URLs will not work.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin