I did restart the nginx service yesterday afternoon but it was only down for 20 seconds max. It is currently up and running.
Are you able to add the apt repository and update from it?
EDIT: to be clear, debian.neo4j.org/repo isn't a webpage and it's expected to return a 404 message if you go there in a browser. If debian.neo4j.org is down then that would definitely be a problem.
The apt repository serves files from debian.neo4j.org/repo and so long as
apt can successfully communicate with that endpoint, it's all fine.