Until today, whenever I double-clicked a photo while in Library mode I got to Develop mode for that photo. Now when I double-click I'm still stuck in Library mode, now the only difference being I see just the one photo rather than many. I have made no updates to the software and am speculating perhaps I changed some setting inadvertently by hitting a key combination. How can I get my old behavior back? I understand I can get there by hitting "D," but I would also like my old behavior all the same. Thanks!
This is the default behavior and I don't remember it being as you described - I use LR since version 1.
You can try to customize a key function: Lightroom – Customizable Keyboard Shortcuts « Julieanne Kost's Blog
I agree with the above - in 7 years of using LR in different Windows OSs, a double click on a thumbnail in the Library grid has always opened that image in Library/Loupe view, not Develop. Possibly you were previously editing using Library/Quick Develop without going into the full Develop module.
To go to Develop, as well as the D shortcut you can either use Window/Develop or Ctrl+Alt+2 (which I have never understood because D is much easier).