Please create a new catalog, import few images and check for the issue.
If this works fine, follow the steps provided in the below link.
Let me know if this helps.
I'll try that but shouldn't I see some reference to Lightroom CC or something other than just a folder called Adobe Lightroom in my Applications folder? I bought the student Photography package and there is the folder Adobe Photoshop CC 2015.
I have Adobe Photoshop Lightroom 4.app in the Applications folder which I now can't open without finding the license number.
Thank you. Very helpful links in the article you sent. I set Lr preferences to "prompt me" rather than always open the last catalog or a specific catalog for now, so I know where I'm working. I'll try to re-import photos from a different catalog and see if that works. I'll try one photo! .
Adobe Lightroom folder, in your Application folder is Lightroom CC 2015 application.
If you are no longer using Lightroom 4, you may remove it by moving it to the Trash.