There is a separate startup preferences file that holds the list of most recent catalogs. It's in the the Lightroom folder that's accessed from Preferences>Presets>Show Lightroom Presets Folder. It's the .agprefs file in the Preferences folder contained within that Lightroom folder. Perhaps something has happened to it? It's only a text file, so you can open it and read it.
You can delete it, but I'd say restart the computer if it's a Mac as preferences now stay in memory. Just remember to start Lightroom by double clicking on the catalog you want to load as deleting those prefs make Lightroom forget what catalogs have been accessed.