You can also try safe mode - go to your site URL with /safe-mode appended to the URL. The logs you can look for at /logs. You can also try Announcing Discourse Doctor from the command line to look for trouble. Sometimes just rebuilding a second time from the command line brings the site back up.
The message you are seeing there is intentionally unspecific so it doesn’t cause alarm for your community members or confuse them. You are right to come here to ask for guidance on troubleshooting steps - in short order you will learn them and know what to do to figure things out.
Good luck and have fun! 