Several possibilities come to mind.
The site may have a limit on the number of users that can simultaneously view or log in at one time to limit denial of service attacks.
Cookies are blocked on your computer, so the site refuses to load for "security" or monitoring reasons.
The site was temporarily off line for maintenance.
The site or server was overloaded with requests.
The Internet was broken.
Your user account was blocked by systems admin.
Browser settings unacceptable for this site.
Could be any one of a number of other reasons. No real difference to dialling a phone number and getting an engaged tone. You try later and get connected.