The owner of nHentai has deleted the website's DNS records on the registrar's side (not deleted on CloudFlare's side).
This action is probably due to the legal action they are facing, which will cost them $4 million in compensation.
To access nHentai, you can specify a manual address for the website by:
Open the file C:\Windows\System32\drivers\etc\hosts with admin rights.
Add the content below at the end of the file:
104.27.206.92 nhentai.net
104.27.206.92 static.nhentai.net
104.27.206.92 t1.nhentai.net
104.27.206.92 t2.nhentai.net
104.27.206.92 t3.nhentai.net