Last week, I had additional options for basemaps in ArcGIS Pro - many of the basemaps offered in ArcGIS Online, such as watercolor, newspaper, human geography, etc. Now, they've gone missing. I'm in the same project file, and I still have one of those basemaps active and it's working; if I select another one, though, I wouldn't be able to get it back.
At some point, I remember seeing a message that I wasn't signed in but now I am. Am I able to access these by signing out? Is that the problem? How can I enable them while signed in?
You can find all ArcGIS Online Basemaps here (53 of them) and add them directly to your ArcGIS Pro project by clicking the three dots far right of each basemap and select "Open in ArcGIS Pro" - this will add them to your project for use. Or in the Catalog Pane in ArcGIS Pro, go to your Catalog Pane, select the Cloud (ArcGIS Online) and search on owner:Esri "web map" - you'll see all the basemaps here to add to your project too! Happy mapping!
By chance are you using a different logins to ArcGIS Online in ArcGIS Pro?
Typically, the basemap gallery items are determined by one's ArcGIS Online administrator. See the images below and are set in Esri Default.