Updated from 2.0.3 to 2.0.4 - just an empty screen

OK. I'll provide the instructions for the alternative fix:

  1. Close the Arduino IDE window if it is running.
  2. Delete the folder at the following path:
    C:\Users\***my_user_name***\AppData\Local\Arduino15\packages\teensy\
    
    :warning: Please be very careful when deleting things from your computer. When in doubt, back up!
    :exclamation: If looking for it with your file browser or command line, note that the AppData folder is hidden by default. On Windows "File Explorer", you can make it visible by opening the "View" menu, then checking the box next to "☐ Hidden items".
  3. Start Arduino IDE.

The IDE should start successfully this time.


The instructions above uninstalled the Teensy boards platform. If you need that platform, you should now reinstall it:

  1. Select File > Preferences from the Arduino IDE menus.
  2. Enter the following URL into the "Additional Boards Manager URLs" field:
    https://www.pjrc.com/teensy/package_teensy_index.json
    
    :exclamation: If there are already Boards Manager URLs in the field, separate them with commas.
  3. Click the OK button.
  4. You will now see a "Downloading index: ..." notification at the bottom right corner of the IDE window. Wait for that notification to close.
  5. Select Tools > Board > Boards Manager from the Arduino IDE menus to open the "Boards Manager" view in the left side panel.
  6. Scroll down through the list of boards platforms until you see the "Teensy by Paul Stoffregen" entry.
  7. Click the "INSTALL" button at the bottom of the entry.
  8. Wait for the installation to finish.

You should now see the boards of the installed platform under the Tools > Board menu in Arduino IDE.