There are reasons when one stop the content in SharePoint from central admin / powershell.
Main reason, when a content database is becoming huge in size then admin stop the content database & create a new content database for new site provisioning. it could also be possible that during the upgrade SharePoint stop it but failed to bring it online.
There are couple of issues when db marked as stopped.
- No new site will provision in this db
- User Profile Syncing with the content database will stop that's mean no profile updates from UPA to User Information list in dbs.
- if you run get-spcontentdatabase then stop database will not be listed.
their is no way to avoid it as SharePoint administrator can do it, but you can use a monitoring tools to monitor your db or you can write your own script which will check the DB status & email/ update a list if anyone stopped.