Surfing on internet on wifi and suddenly safari can not load webpage - disconnect & connect and everything is ok

Power off the router. Unplug it from the wall. Wait a while.

Plug it back in. Power the router on. Wait until all lights are lit properly. It will take a while.

Reset your Network Settings.

Tap Settings > General > Reset > Reset Network Settings.

This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before.

https://support.apple.com/en-us/HT201415

Restart your device.  https://support.apple.com/en-us/HT201559

Safari can’t open a website

 Safari – slow page loads

 Safari - Unsupported third-party add-ons may cause Safari to unexpectedly quit or have performance issues

Safari doesn't load a page or webpage items are missing

Web Content not responding

Sometimes, just clearing the cookie log jam, or the browser cache can resolve inability to load a web page.

1. Safari Preferences : Privacy : [Manage Website Data…]
2. Safari Preferences : Advanced panel : [√]Show Develop menu in menu bar
   1. Use option+command+E keyboard shortcut to clear browser cache
   2. The "Develop" menu bar item will flash a blue background when using the keyboard shortcut