Double check if you have entered the correct URL.
If the URL has been inputted correctly, try another device to open the same URL. Some websites may be inaccessible because they are down, so you can check it by accessing from another device.
Also check your device’s internet browser settings.
If you have installed an Anti-virus app or software, this may prevent you from accessing specific websites. You should also check if such is not hampering your access.
Perform a trace route.