This is most likely caused by a corrupt preferences file. The location of this file is:
~/Library/Preferences/com.vernier.loggerpro.plist

Follow these steps to reset the preferences file:

1. Quit Logger Pro.
2. Go to Finder and open the Go menu.
3. While holding the Option key, choose Library from the Go menu. (Note: Library does not show up in the Go menu unless you are holding down the Option key.)
   
4. Open the Preferences folder.
   
5. Search the folder for “vernier” to find the com.vernier.loggerpro.plist file. Move or copy the file to your desktop (as a backup).
   
6. If the file still exists in your Preferences folder, move it to the trash.
7. Restart your Mac.
8. Launch Logger Pro.