I found a solution that worked from the Arduino.cc forum. It is reprinted below. The solution is to delete 2 folders found in your ‘User’ directory and ‘\User\AppData\roaming’ directory in Windows, and then re-start Arduino IDE. It might also be that the folders have the ‘read-only’ attribute set, at least it was set on my folders. It seems this problem has occurred for many versions of Arduino IDE, not just 2.3.3. It just happened to occur for me when I upgraded to 2.3.3.

Hi @hannmend. It is possible the problem is caused by some problem in the Arduino IDE application configuration data. If so, it should be possible to restore the IDE to functionality by deleting the folders where the data is stored. This will force Arduino IDE to automatically regenerate the default configuration data.
Please try this:

 The folders to be deleted do not contain any of the data for your sketch projects so deleting them should not result in the loss of any irreplaceable data. However, please always be careful when deleting things from your computer. When in doubt, back up!

1. Close all Arduino IDE windows if the IDE is already running.
2. Delete the folder at the following path:

   C:\Users\<username>\.arduinoIDE\
   

   (where <username> is your Windows username)

3. Delete the folder at the following path:

   C:\Users\<username>\AppData\Roaming\arduino-ide\
   

   (where <username> is your Windows username)
    If looking for it with your file manager 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“.

Now start Arduino IDE again. Hopefully this time it will start up fully.

 The Arduino IDE preferences will have been reset to their default values. If you had previously made any customizations to the preferences (e.g., the “Additional Boards Manager URLs” preference) then you will now want to reconfigure the IDE as necessary.

Hi.
Thanks for sharing this solution.
I was also having a similar problem but with the ESP32 downloads.
Thank you.
Regards.
Sara

UPDATE and ADDITIONAL SOLUTION: I had this problem again, so I deleted the Arduino directories as above but that did not resolve the issue. So I asked Microsoft Copilot and it gave the solution of closing the serial monitor when libraries are being downloaded. This worked. As soon as I closed the Serial Monitor the downloads resumed and completed. According to Copilot, this is a bug in version 2.3.3.

Thanks for sharing the solution.
I’ll mark this issue as resolved. If you need further help, you just need to open a new question in our forum.
Regards,
Sara