It's possibly a problem with the domain name system server used by your ISP. Try using a different DNS server:
Click Start
Click Network & Sharing Centre
In the right-hand section, under 'View Your Active Networks', click whatever appears next to 'Connections'
Click Properties
Click on Internet Protocol Version 4 (TCP/IPv4)
Click Properties
Click to select 'Use the following DNS server addresses'
Enter these numbers in the top row: 216 . 146 . 35 . 35
Enter these numbers in the bottom row: 216 . 146 . 36 . 36
Click OK
Click Close
Click Close
Close the window
You'll then be using the free DNS server provided by Dyn (which is what I'm currently using). It might be more reliable than the DNS server that your ISP normally routes your traffic through. It certainly won't do any harm to give it a try. (There are loads more free DNS servers listed here if you ever need to switch:
https://www.lifewire.com/free-and-public-dns-servers-2626062 )