Switching between multi-monitor and single-monitor display settings if one of the windows is located on the display that is no longer present can cause the issue you are describing. Please try the following and let us know if these steps address the issue you are experiencing:
Quit Santos Pro (critical 1st step)
Locate the file, layout.config, which can be found in your \\Documents\\SHI\\Santos directory
Delete layout.config
These first 3 steps are critical because the layout.config file is updated with the current (potentially problematic) window settings when you exit the application
If the layout.config file does not exist when Santos Pro is restarted (this is what we want to happen to address this issue), the software writes out the default factory windows layout settings and uses those on startup
Restart Santos Pro