It was fine yesterday. It looked like this:

Today, it looks like this:

with just that little tiny shrunk spot being my logo up there where it belongs. It's clickable, and goes to the main site as usual. When right clicked and open image in new tab, it shows up the proper size.
Have no clue what's going on. May have been the result of a plugin I tried, I suppose, because I was messing with them at some point this week, but I've uninstalled and/or deactivated anything that isn't a plugin that I've been using for a good long while.
Help and suggestions greatly appreciated. I'm at my wits end looking for the cause.