Second, be patient, try the site tomorrow. Not every site has a rock-solid connection and yours may not be perfect either.
If that's not it, then unfortunately the site is probably really gone. Sites go out of business all the time or forget to renew domain names, and poachers grab popular names to advertise competing products or even completely unrelated garbage. The prior owners of the site can fight in civil court if they have a trademark, but it's not easy.
Sometimes a site will recover from such a disaster under a new name, or even sell a name intentionally to raise money for their purposes. Try a Google search for the subject matter of your site. You may discover their new location -- if they have one. Good luck!
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