If it’s down for everyone you have a website hosting issue. I usually give it 10-15 minutes or so to resolve itself before I make a call to the hosting provider.
If it’s just unavailable to you then you probably have a local Internet connection issue. If that’s the case, restart your computer and see if the connection issue goes away – most times it does. You might also power-off and power-up (cycle) your Internet router. If either of these things don’t resolve the issue it’s time to call your Internet service provider.