Foolishly I tweaked Unity Tweak Tool and froze my Ubuntu windows. How can I unfreeze my windows to activate Ubuntu programs without a complete reinstallation of Ubuntu?
From your description the user password doesn't work, the GUI user interface doesn't work and it isn't even possible to login in a text-only console. This could be caused by a hardware problem instead of a software problem. If so, reinstalling Ubuntu won't fix anything, but it is still best to try a software solution first.
You may be able to install and access the Unity Tweak Tool application (unity-tweak-tool) after logging in to a Guest session. Every time you click on an icon in the Unity Tweak Tool interface it changes to a new screen that has a Restore defaults button at the bottom of the screen. There are 20 icons in the Unity Tweak Tool, so you can restore all the defaults by clicking the Restore defaults button for each of the 20 icons.
How do I reset a lost administrative password? tells how to reset the user password so that you can login in a text-only console.
How to switch to a command-line console tells how to switch to a text-only virtual console that doesn't have any windows. You can reinstall Unity Tweak Tool from the console with the command:
sudo apt-get install unity-tweak-tool. You can also skip reinstalling the Unity Tweak Tool because maybe it won't work if the Unity Tweak Tool interface is frozen, and instead reinstall the entire Ubuntu desktop:
sudo apt purge ubuntu-desktop sudo apt install ubuntu-desktop sudo reboot