Visit the rental story map to get all the basics on collecting data, navigating, and transferring data using TerraSync and Pathfinder Office. https://storymaps.arcgis.com/stories/6400358a8a3b4fa99a350b119cd903f2
It's not really locked up. The window is just out of view of the user. Same thing can happen with Data Transfer. If you lose a window like that out of Pathfinder Office in the future, remember this little trick: Hold down Alt + the space bar,and ...
This was a bug in TerraSync version 5.8 where the software would crash when trying to save a feature while a data file was open as a background file. upgrading to 5.81 or later will fix the issue.