I recently had the problem that my SOLR Service was not starting all of a sudden. So I searched the internet and asked colleages but I had quite a hard time figuring out what the issue was. So here is a summary of possible reasons that you can check.
Do you have agreed on SLAs towards your website and authoring environments? When thinking about your Sitecore Infrastructure you have many opportunities to scale out. Let's take a look what we considered in our project.