In regards to changing name servers for domain names, it will usually take around 15 - 30 minutes for the name servers to propagate (definition: to spread and travel). This means when you set or change name servers for domain names, it may take a certain amount of time for ISPs around the world to refer to the new name servers instead of referring to the old ones in their cache (cache means "for storing or future use" - pronounced as "cash", it basically is a database of name servers so the ISP doesn't need to request the name server each and every time, so to speak).
That said, if it takes longer than 15 - 30 minutes, it may take a few hours for propagation. It is not rare to have to wait 24 to 72 hours for propagation to complete fully (or if you are specifically experiencing problems accessing your website via your ISP). Usually it takes 30 minutes, however

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Once you change the name servers, you may want to wait 30 - 60 minutes then try accessing your website. If others can access your website but you still cannot access your website, you may want to try resetting/restarting your router/modem for your Internet access as because you have accessed the site before the reason you still cannot access the website may be down to local cache on your side.