When you are having problems visiting websites, it could be one of a number of reasons preventing you from connecting. Here are a few simple things you can check.
First off, check your router/gateway/hub/whatever you want to call it. It may have developed a problem that can easily be fixed by restarting it (a cliché yes, but it can work) which clears its internal memory. While doing that it can also help if you restart your computer for similar reasons.
While checking your hub, make sure you have a green internet light. If it is red or not lit at all, then your ISP may very well have a problem their end. Before ringing them though, check all of the cables are seated properly to and from your hub and the wall socket.
Next it is time to take a closer look at your PC. On Windows you can try using the troubleshooting software that is built in to the operating system. Right click on the network icon in the system tray (usually in the bottom right of the screen, it may look like a cable plugged in or a WiFi signal strength icon and if there is a problem it may have a yellow triangle with an exclamation mark) and select the troubleshoot connection option and follow the prompts. That will perform various tests and make changes for you if it finds any problems.
Still got problems? It’s time to check third party software. Antivirus software often incorporates a firewall. Firewalls are designed to filter out unwanted traffic getting into your machine, but can also work the other way too by blocking traffic coming from your machine. Sometimes things go wrong. How you check your firewall and make appropriate changes will depend on the software you use. If in doubt check with the manufacturer.
Browser extensions and plugins can and do interfere with performance and the display of websites. In extreme cases they can even stop pages loading completely. The only way to find out if one of these is the cause of your problems is to temporarily disable and/or uninstall the plugin.
One last thing to check is your computer hardware itself. There may be a driver issue which can be resolved by installing an updated driver, or you may have faulty hardware.