Changes to the settings of a domain name, including DNS servers, may take up to 24 hours to become effective. You may check in which areas the DNS propagation has been completed here. When the propagation is complete, you will be able to access your website from the server which hosts your account.
If after 24 hours the DNS change has not taken effect yet, you may try clearing your browser’s cache. You can read how to do that from here.
If you would like to skip the propagation period, you may configure your domain to resolve to a certain IP address. For that purpose, you may modify your hosts file.