Try with your Firewall switched off. And check to see if any of your Anti-malware programs are blocking it. Try using another Browser and see if that works. Try Command Prompt and type in ipconfig /flushdns and press enter. it should tell you DNS cache has been flushed if so reboot and see if that works. Sometimes it is an ISP problem and it comes back eventually. Or type the address on here it will tell you if it is just you.