You’ve likely tried to open an image with LR instead of starting up LR and importing the image into LR. The drag-and-drop target file type for LR is a catalog not an image so if you drag-and-drop an image onto LR it’ll create a catalog with that name and not know what to do with it since it is an image.
To fix things, hold down the Alt key while you start up LR and it’ll ask what catalog to open then pick another one, or create a new catalog if this is your first time starting LR.
And if ssprengel's fix doesn't work (which I suspect, from previous testing, that it might not) the next thing to do is to reset the preferences file. See this link for instructions:
I figured it out...turns out I had not closed out of the file properly before I unplugged the external drive it was on. I had to go in and make the file a copy with a new name and then delete the original. When I did that all was well.