Sitecore uses indexes to store lookup information in an quick and easy to search format, to allow for fast searches and lookups. Depending on how it is configured, Sitecore 9 relies on a SOLR server to store and run its indexes.
I came across an issue not too long ago that was very frustrating to solve. It had to do with an automatic Java update. You know, those little popups that show up in your lower right corner every so often. Well, I saw this popup and clicked on it, not thinking anything of it. When it came time to continue working on my local Sitecore site, I saw that my site wasn't working correctly.
Long story short, after some frustrating debugging, I finally narrowed it down to the JAVA_HOME environmental variable. It needed to be updated to point to the new Java location, that was changed during the install.
To get to the environmental variables window, type "view advanced system settings" into your Windows start menu. Click on the "Environmental Variables" button. There, you will see a JAVA_HOME variable with a value. The value needs to point to the correct path of your Java install folder. Once that is updated, go to your Windows Services and make sure your SOLR service is running, then visit the URL of your SOLR server to make sure it all works.
The takeaway is: Be aware that your SOLR server might go down after a Java update. I hope this post helps someone!