Setting up a new website, and just pointed your domain’s nameservers to your hosting provider?
It takes up to 48 hours for DNS changes to propagate, and sometimes you don’t want to wait that long. In general, I recommend you do wait, because specially in the first few hours after a change, things might be funky. Your domain might resolve with the correct nameservers one minute, and the next it doesn’t. (I don’t know the technical reasons why that’s the case—it doesn’t seem to make sense at first, given that this happens even when you’re accessing the same URL from the same network, but I’ve witnessed it often enough to be true.)
However, if you don’t want to wait, you can check the propagation status of your websites with this neat little free online tool: WhatsMyDNS
WhatsMyDNS is a global DNS propagation checker, aka a DNS testing tool.
The site also offers more explanations around how domain name servers work and so on, but their propagation checker is reall what I go there for.
I sometimes get false negatives here though.
I noticed too that even long-propagated domains sometimes show up als not yet propagated in certain localities, not sure why that is. Still, while not 100% it’s a useful tool for me. (plus, it’s free)