Could be a DNS or IP blackhole created by your ISP or locality. There are several things you can try:
1. From the FAQ you can try:
I am getting blocked because of my school/work filters. Is there a way around them?
You can try to connect directly via our IP addresses using a special port:
https://208.100.40.128:444/index.htm
but your browser (and the filter you are trying to bypass) won't be able to verify the site identity, requiring you to go to advanced options and continue regardless.
2. You should change your DNS to Cloudflare. Go to
https://1.1.1.1/ and scroll down for PC instructions.
3. Your firewall may be misconfigured. You can try temporarily disabling to see if that fixes it:
https://www.computerhope.com/issues/ch000551.htm
4. You can setup your computer to connect to the internet via a VPN which will tunnel the connection to bypass any filters in place.