I just installed a fresh AWS marketplace image.
Brought the whole system up without any problem. seemed to be working fine.
SSH window suggested a reboot was necessary.
So I rebooted the server.
Now Nextcloud is not running.
This is my third attempt at starting one of these servers.
The first time I had trouble with eternal storage.
Second time it crashed when I attempted to map the data to a second ebs volume.
This time it seems the service is just not restarting.
I found a Stack article that looked promising here…
But none of the options in that post worked.
Apache and mysql seem to be running ok.
Is there a service that does not auto start that I need to start?
so NextCloud by default sets the mysql user oc_admin.
which is not what the instructions say to do.
You need to provide some more information 
:
As you say AWS marketplace image and give link to an ubuntu/mariadb issue, is assume you use this image? Is there any difference/preconfiguration compared to the ubuntu lts officially provided by ubuntu?
Did Nextcloud run before you did the first reboot?
Did you check service apache2 status and service mysql status to check if both is running fine and also checked error logs for both?
Which instructions do you refer to? Of course you need to create this user within mysql/mariadb, or create one with name of your wishes and change the config.php accordingly/better set this directly on first nextcloud page load for installation. See the admin manual about this: Database Configuration — Nextcloud 12 Server Administration Manual 12 documentation
Generally would be fine to have a look at your config.php, apache and in case php configurations.