If you're account and/or domain is new, or you recently made DNS changes, your domain name may not have propagated through the system yet. This usually takes about 4 - 24 hours, while sometimes as soon as 15-30 minutes.
If you were able to access your site before, try to pull it up via your Temp URL. This is the URL listed in cpanel i.e. server-ip/~username .
Few other possible reasons:
1. Your domain name may have expired.
2. Your domain was suspended by domain registrar due to failed verification .
3. Your domain name may not be pointed to your hosting server's nameservers .