I missed an email from my domain provider requesting I verify my contact details in accordance with ICANN's policies, this resulted in a landing page being placed on my domain root. The page said after verification the site would return after 24 to 48 hours. The landing page remained in place for approx. 6 days, until I made DNS changes.
"Reset" your website's DNS. i.e., change the IP addresses the A records point to. In my case this was GitHub Pages IP addesses. After just a few minutes the website returned. I'm not sure if the IP addresses were changed in the first place, but this worked for me.